\AI 


& 


COLOR 

AND    ITS    APPLICATIONS 


BY 

M.   LUCKIESH 

DIRECTOR  OF  APPLIED  SCIENCE,  NELA  RESEARCH  LABORATORIES 
NATIONAL  LAMP  WORKS  OF  GENERAL  ELECTRIC   CO. 

AUTHOR  OF  "  LIGHT  AND  SHADE  AND  THEIR  APPLICATIONS,"    "  THE  LIGHTING 

ART,"  "  THE  LANGUAGE  OF  COLOR,"  "ARTIFICIAL  LIGHT,  ITS  INFLUENCE 

UPON  CIVILIZATION,"  "  LIGHTING  THE  HOME,"  ETC. 


150  Illustrations— 4  Color  Plates  —  34  Tables 


SECOND  EDITION 
ENLARGED 


NEW  YORK 
D.  VAN   NOSTRAND   COMPANY 

EIGHT  WARREN  STREET 
IQ2I 


CQPYRIGHT     1915,     IQ2I,     BY 
D.   VAN    NOSTRAND     COMPANY 


THE     PLIMPTON     PRESS 
NORWOOD 'MASS 'U'S 'A 


PREFACE 

The  aim  of  this  book  is  to  present  a  condensed 
treatment  of  the  science  of  color.  An  attempt  has 
been  made  to  cover  as  many  phases  of  the  subject 
as  possible  within  the  confines  of  a  small  volume. 
During  several  years  of  experimental  work  in  the 
science  of  color  I  have  been  brought  into  contact  with 
many  persons  interested  in  its  applications,  and  the 
desire  has  been  frequently  expressed  for  a  book  that 
treated  the  science  of  color  as  far  as  possible  from 
the  viewpoint  of  those  interested  in  the  many  applica- 
tions of  color.  These  applications  are  constantly  in- 
creasing in  scope  and  interest.  With  this  viewpoint 
in  mind  I  have  attempted  to  treat  the  subject,  exercis- 
ing my  judgment  in  drawing  freely  from  the  work  of 
other  investigators  in  order  to  make  the  volume  as 
comprehensive  as  possible.  I  do  not  feel  that  the 
work  comprises  a  complete  treatment,  for  there  are 
many  interesting  phases  of  color  science  that  have 
been  barely  touched  upon,  and  some  that  have  been 
purposely  omitted,  because  of  the  danger  of  straying 
too  far  afield.  It  is  believed,  however,  that  this 
treatise  will  be  helpful  to  those  interested  in  any  of 
the  arts  involving  the  science  of  color.  I  have 
referred  to  my  own  investigations  quite  freely,  but 
trust  that  this  will  not  be  attributed  to  a  lack  of  per- 
spective. Naturally  much  of  the  text  involves  my 
own  conclusions,  but  I  have  aimed  to  include  only 
those  that  are  supported  by  experimental  data,  be- 
cause only  in  so  far  as  they  are  thus  supported  does 

iii 


520416 


iv  PREFACE 


the  work  become  authoritative.  Many  unsolved  prob- 
lems have  arisen  throughout  the  text,  which  em- 
phasizes the  need  for  more  workers  in  the  field.  No 
attempt  has  been  made  to  present  a  complete  bib- 
liography of  even  the  recent  work  in  this  branch  of 
science ;  but  references  have  been  given  freely,  which, 
if  followed,  will  provide  a  substantial  beginning  to  the 
almost  endless  chain  of  material  available. 

It  is  a  pleasant  duty  to  record  my  acknowledg- 
ments to  the  management  of  the  National  Lamp 
Works  of  the  General  Electric  Company,  whose  broad- 
minded  spirit  in  establishing  the  Nela  Research 
Laboratory  has  made  this  work  possible,  and  to  the 
director  of  the  laboratory  and  members  of  the  staff, 
who  always  have  given  freely  of  their  time  and 
counsel. 

SECOND   EDITION 

« 

Some  changes  have  been  made  in  the  original  text 
and  an  extensive  chapter  has  been  added.  This  con- 
sists of  useful  data  and  methods  for  their  use. 

M.  LUCKIESH 
September,  1920 


CONTENTS 

CHAPTER   I 

Page 
LIGHT 1 

Wave  Theory.  Electro-magnetic  Theory.  Radiation  and  Light  Sensa- 
tion. Temperature  and  Radiation.  Spectra  of  niuminants. 

CHAPTER   II 

TUB  PRODUCTION  OF   COLOR 23 

Refraction.  Diffraction.  Interference.  Polarization.  Reflection,  Ab- 
sorption, and  Transmission.  Color  of  Daylight.  Color  Sensations 
Produced  by  Colorless  Stimuli.  Fluorescence  and  Phosphorescence. 
Useful  Filters. 

CHAPTER   III 

COLOR-MIXTURE 64 

Subtractive  Method.  Additive  Method.  Juxtapositional  Method. 
Simple  Apparatus  for  Mixing  Colors. 

CHAPTER   IV 

COLOR  TERMINOLOGY 69 

Hue,  Saturation,  and  Brightness.    Tri-color  Method.    Color  Notation. 

CHAPTER   V 

THE  ANALYSIS   OF  COLOR 86 

The  Spectroscope.  The  Spectrophotometer.  The  Monochromatic 
Colorimeter.  The  Tri-chromatic  Colorimeter.  Other  Methods. 
Templates.  Reflectometer.  Methods  of  Altering  Brightness  Non- 
selectively. 

CHAPTER  VI 

COLOR  AND  VISION fl6 

The  Eye.  Brightness  Sensibility.  Hue  Sensibility.  Saturation  Sensi- 
bility. Visual  Acuity  in  Lights  of  Different  Colors.  Growth  and  Decay 
of  Color  Sensations.  Signaling.  Other  Uses  for  Colored  Glasses. 


vi  CONTENTS 

CHAPTER  VII 

THE   EFFECT   OF  ENVIRONMENT   ON   COLORS ' 163 

Illumination.    After-images.    Simultaneous  Contrast.    Irradiation. 


CHAPTER   VIII 
THEORIES   OF   COLOR  VISION.. 


Young-Helmholtz.  *  Duplicity.'  Hering.  Ladd-Franklin.  Edrigde- 
Green. 

CHAPTER   IX 

COLOR  PHOTOMETRY 191 

Methods  of  Color  Photometry.  Other  Means  of  Eliminating  Color  Dif- 
ferences. Direct  Comparison  and  Flicker  Methods.  Luminosity 
Curve  of  the  Eye. 

CHAPTER   X 

COLOR  PHOTOGRAPHY 213 

Lippmann  Process.    Wood  Diffraction  Process.    Color  Filter  Processes. 


CHAPTER   XI 

COLOR  IN  LIGHTING , 224 

Artificial  Daylight.  Units  for  Imitating  Daylight.  Effect  of  Colored  Sur- 
roundings. Color  in  Interiors.  Color  Preference.  A  Demonstration 
Booth. 


CHAPTER   XII 

COLOR  EFFECTS  FOR  THE  STAGE  AND   DISPLAYS 272 

Stage.     Displays. 

CHAPTER   XIH 

COLOR  PHENOMENA  IN  PAINTING 282 

Visual  Phenomena.    Lighting.    Pigments. 

CHAPTER   XIV 

COLOR  MATCHING 302 

The  Illuminant.     The  Examination  of  Colors. 


CONTENTS  vii 

CHAPTER   XV 

THE  ART  OF  MOBILE  COLOR 312 

Color  Music.    Its  Relation  to  Sound  Music. 


CHAPTER    XVI 

COLORED  MEDIA 327 

Available  Coloring  Materials.  Dyeing.  Gelatine  Films.  Solvents. 
Lacquers.  Celluloid.  Phosphorescent  Materials.  Miscellaneous 
Notes. 

CHAPTER   XVH 

CERTAIN   PHYSICAL  ASPECTS   AND   DATA 344 

Three  Types  of  Colored  Media.  Pigments.  Optical  Properties  of  Pig- 
ments. Applications  of  Spectral  Analyses  of  Pigments.  Reflection- 
factors  of  Pigments.  Spectral  Analyses  of  Dye-solutions.  Applications 
of  Spectral  Analyses  of  Dyes.  Laws  Pertaining  to  Colored  Solutions. 
Dichromatism.  Graphical  Method  for  Using  Spectral  Data.  Spectral 
Analyses  of  Glasses.  Red,  Yellow,  Green,  Blue,  and  Purple  Glasses. 
Use  of  Spectral  Analyses  of  Glasses.  Influence  of  Temperature  on 
Transmission  of  Colored  Glasses.  Ultraviolet  Transmission  of  Media. 
Compounds  Sensitive  to  Temperature.  Transmission  of  Light  by  Fog 
and  Water.  Color  Temperature  of  Illuminants. 

INDEX  .  407 


COLORED    PLATES 

Prismatic  Spectrum Frontispiece 

Diffraction  Grating  Spectrum " 

Subtractive  method  of  mixing  colors Facing  page    64 

Additive  method  of  mixing  colors "          "      54 

Showing  the  effect  of  environment  on  the  appearance  of  colors         "          "    163 
Illustrating  the  effect  of  the  spectral  quality  of  the  illuminant 

Daylight,  below ;  ordinary  artificial  light,  above    .   .          "          "    282 


LIST  OF  ILLUSTRATIONS 

Figure  Page 

1.  Radiation  curve  of  an  incandescent  solid 8 

2.  Showing  the  relation  between  radiant  energy  and  light  sensation ...  10 

3.  Showing  the  effect  of  temperature  on  the  radiation  from  an  incandes- 

cent solid  (black-body) 12 

4.  Representative  spectra 17 

6.   Distribution  of  energy  in  the  visible  spectra  of  various  illuminants .    .  20 

6.  Newton's  experiment 23 

7.  Effect  of  the  character  of  the  slit  of  a  spectrograph  on  the  grating  spec- 

trum of  the  mercury  arc 24 

8.  Dispersion  curves  of  various  optical  media 25 

9.  Young's  double-slit  experiment  illustrating  the  principle  of  the  diffrac- 

tion grating 26 

10.  Diagrammatic  illustration  of  polarized  light 31 

11.  The  Nicol  prism  for  obtaining  plane-polarized  light 33 

12.  Analyses  of  ordinary  colors 36 

13.  Showing  the  variation  in  the  spectral  character  of  sunlight  due  to  at- 

mospheric absorption 38 

14.  Benham  disk  for  producing  subjective  colors  by  means  of  black  and 

white  stimuli 39 

15.  Diagrammatic  illustration  of  the  action  of  the  rhodamine  fluorescent 

reflector 44 

16.  Spectrophotographic  analysis  of  the  action  of  the  rhodamine  fluores- 

cent reflector 45 

17.  Screens  for  producing  lights  of  the  same  hue  but  differing  in  spectral 

character 48 

18.  Ultra-violet  spectra 60 

19.  Ultra-violet  spectra 51 

20.  The  subtractive  method  of  mixing  colors  (colored  plate) 

21.  The  additive  method  of  mixing  colors  (colored  plate) 

22.  The  color-wheel  for  showing  complementary  hues ' .    .  59 

23.  Maxwell  disks 62 

24.  An  erratic  color-mixing  disk 64 

25.  A  simple  color-mixer 64 

26.  A  simple  color-mixer  for  transparent  or  opaque  media 65 

27.  Lambert's  color-mixer 65 

28.  A  shadow  demonstration  of  the  additive  and  subtractive  methods  of 

color-mixture 66 

29.  Illustrating  a  disk  for  approximating  a  prismatic  spectrum 68 

30.  Disk '  a,'  for  varying  only  the  saturation  of  a  color.  —  Disk  *  b,'  for  vary- 

ing only  the  brightness  of  a  color 71 

31.  The  Maxwell  color-triangle 73 

32.  Spectral  complementaries 75 

33.  A  color  pyramid 75 

34.  The  double  pyramid  (after  Titchener) 76 

36.  A  demonstration  color-triangle 76 


LIST   OF   ILLUSTRATIONS 


36.  The  A.  H.  Munsell  color  tree "...  81 

37.  Prang's  color  and  brightness  scales 82 

38.  Ruxton's  color  mixture  chart  for  printing  inks 82 

39.  A  direct-vision  prism  spectroscope 86 

40.  A  simple  grating  spectroscope 86 

41.  The  spectrophotometer 88 

42.  The  Nutting  pocket  spectrophotometer 88 

43.  A  small  portable  spectrophotometer  for  quantitative  analysis   ....  89 

44.  The  variable  sectored  disk  (after  Hyde) 90 

45.  Scheme  for  reducing  the  amount  of  spectrophotometric  work  in  ex- 

amining transparent  colored  media 91 

46.  Abney's  spectrophotometric  attachment  for  a  spectrometer 93 

47.  Ives'  spectrophotometric  attachment  for  a  spectrometer 93 

48.  Nutting's  spectrophotometric  attachment  for  a  spectrometer    ....  94 

49.  The  Nutting  monochromatic  colorimeter 96 

60.  Analysis  of  two  component  color-mixtures 99 

61.  A  simple  method  of  converting  a  spectrometer  into  a  combined  mono- 

chromatic colorimeter,  direct  comparison  photometer,  flicker  pho- 
tometer, and  spectrophotometer 100 

52.   Illustrating  the  principle  of  the  Maxwell  '  color  box  ' 101 

63.   The  F.  E.  Ives  colorimeter 103 

54.  Kb'enig's  sensation  curves 104 

65.  Tri-color  colorimeter  measurements 104 

66.  Arrangement  for  using  color  filters  before  a  photometer  eyepiece   .    .  106 

57.   Arons  colorimeter 108 

68.  Abney's  template  for  carmine 110 

59.  Adaptation  of  Abney's  scheme  for  the  spectroscopic  synthesis  of  color  111 

60.  The  Nutting  reflectometer 113 

61.  A  vertical  section  of  the  human  eye 116 

62.  Showing  the  effect  of  chromatic  aberration  in  the  eye 118 

63.  A  simple  achromatic  lens 119 

64.  Limits  of  the  visual  field  for  colored  and  colorless  lights 120 

65.  Brightness  sensibility  data.     (See  Table  X) 121 

66.  Hue  sensibility.     (Steindler's  Eye) 125 

67.  Hue  sensibility,  limen,  and  color  scale 126 

68.  Apparatus  for  determining  visual  acuity  in  monochromatic  lights    .    .  133 

69.  Visual  acuity  in  monochromatic  lights  of  equal  brightness    .    .    .    .    .  136 

70.  Visual  acuity  in  the  mercury  spectrum,  the  lines  being  reduced  to  equal 

•brightness 136 

71.  The  growth  and  decay  curves  for  white  light  sensation.    (Broca  and 

Sulzer) 138 

72.  The  growth  and  decay  curves  of  color  sensations   . 139 

73.  Showing  the  maxima  attained  by  flickering  lights  at  various  frequencies  140 

74.  Showing  the  maxima  of  sensations  produced  by  flickering  red  light 

on  a  steady  green  field  (R),  and  vice  versa  (G) 141 

75.  Showing  the  relation  between  brightness  and  critical  frequency  for 

colored  stimuli 145 

76.  Effect  of  contour  of  flicker  on  critical  or  vanishing-flicker  frequency  .  147 

77.  Effect  of  yellow-green  glasses  on  vision  under  a  bright  sky 155 

78.  Ultra-violet  transmission  curves  of  various  glasses 168 

79.  Effect  of  the  intensity  of  illumination  on  the  appearance  of  a  pigment  166 

80.  Illustrating  why  a  purple  appears  differently  under  two   different 

illuminants 167 

81.  Effect  of  brightness  on  the  duration  of  the  after-image 171 


LIST  OF  ILLUSTRATIONS  xi 

82.  Showing  the  effect  of  simultaneous  contrast.    The  V's  are  of  equal 

brightness 174 

83.  Showing  induction.   Each  band,  though  uniform  in  brightness,  appears 

brighter  at  the  right-hand  edge 175 

84.  An  arrangement  for  showing  tjie  reduction  in  the  contrast  effect  by 

separating  the  two  colored  objects 176 

85.  An  arrangement  for  showing  the  effect  of  simultaneous  contrast  and 

after-images 176 

86.  Illustrating  irradiation 179 

87.  The  evolution  of  the  Ladd-Franklin  gray  molecule 187 

88.  The  results  of  four  methods  of  photometry.    (Ives) 195 

89.  Spectral  sensibilities  of  selenium  and  photo-electric  cells  compared 

with  the  spectral  sensibility  of  the  eye 200 

90.  Spectral  sensibility  of  a  panchromatic  photographic  plate 202 

91.  An  accurate  color  filter  for  the  panchromatic  plate  considered  in  Fig.  90.  203 

92.  Results  by  flicker  and  direct  comparison  photometers,  illustrating  dif- 

ferences including  the  Purkinje  effect  and  a  reversed  effect  .    .  206 

93.  Visibility  data.    (See  Table  XVI) 209 

94.  Illustrating  the  standing  waves  produced  in  the  Lippmann  process .    .  215 

95.  Illustrating  the  Wood  diffraction  process 216 

96-98.  Illustrating  three  processes  of  color  photography 219 

99.   Illustrating  the  limitations  of  certain  processes  of  color  photography .  220 

100.  Ideal  transmission  screens  for  producing  artificial  daylight 230 

101.  Showing  the  loss  of  light  when  using  the  ideal  artificial-daylight  screens 

with  the  tungsten  lamp  operating  at  7.9  lumens  per  watt  ....     231 

102.  Showing  the  loss  of  light  when  using  the  ideal  artificial-daylight  screens 

with  the  tungsten  lamp  operating  at  22  lumens  per  watt  ....     232 

103.  Showing  the  spectral  analyses  of  two  subjective  white  lights  com- 

pared with  the  spectral  analysis  of  noon  sunlight 235 

104.  Showing  the  additive  method  of  producing  artificial  daylight    ....     236 

105.  Showing  the  relative  amounts  of  light  of  the  character  of  A  and  B 

(Fig.  104)  necessary  to  produce  artificial  daylight  by  addition  .    .     237 

106.  Illustrating  the  effect  of  multiple  selective  reflection  of  light  from  a 

green  fabric 248 

107.  Showing  the  relative  proportions  of  red,  green  and  blue  components  in 

the  reflected  light  from  a  green  fabric  after  various  successive 
reflections 249 

108.  Screen  for  altering  tungsten  light  to  the  same  spectral  character  as 

carbon  incandescent  electric  light;  c,  d,  e  show  the  transmission 

curves  of  amber  glasses  of  different  densities 254 

109.  Comparison  of  ideal  screen  a,  Fig.  108,  with  amber  glass 265 

110.  Showing  the  preference  or  rank  of  a  number  of  fairly  saturated  colors  .  261 

111.  Wiring  diagram  of  an  experimental  and  demonstration  booth  ....  267 

112.  Showing  dimensions   and  locations  of  lamps  in  the  demonstration 

booth 268 

113.  Illustrating  the  effect  of  colored  light  upon  the  appearance  of  six 

colored  papers 273 

114.  Illustrating  the  changing  of  scenery  by  the  use  of  colored  lights .    .    .     275 

115.  Illustrating  the  disappearing  effects  produced  on  a  specially  painted 

scene  by  varying  the  color  of  the  illuminant 276 

116.  Illustrating  a  flashing  sign  produced  by  properly  relating  the  hue  and 

brightness  of  the  pigments  with  the  color  of  the  illuminant .    .    .     279 

117.  Showing  the  reflection  coefficents  of  fairly  saturated  colors  for  day- 

light and  tungsten  incandescent  electric  light.    (See  Table  XV) .    .     286 


xii  LIST  OF  ILLUSTRATIONS 

118.  Showing  the  effect  of  the  illuminant  upon  the  appearance  of  a  colored 

frieze 288 

119.  Showing  the  effect  of  the  spectral  character  of  the  illuminant  upon  the 

values  of  a  painting 290 

120.  Effect  of  distribution  of  light  on  the  expression  of  a  painting 293 

121.  Illustrating  the  optics  of  picture  lighting 294 

122.  Spectral  analyses  of  pigments 298 

123.  Spectral  analyses  of  pigments 298 

124.  Illustrating  the  effect  of  the  amount  of  the  green  components  in  blue 

and  yellow  pigments  on  the  amount  of  '  black  '  in  the  mixtures  .     299 

125.  Diagrammatic  illustration  of  the  results  of  mixing  blue  and  green  pig- 

ments containing  various  amounts  of  green 300 

126.  The  '  Luce  '  part  for  the  '  Clavier  a  lumieres '  in  Scriabine's  '  Pro- 

metheus ' 315 

127.  Illustrating  an  instrument  for  studying  the  emotive  or  affective  value 

of  colors  and  color  phrases ;  Rimington's  color  code  also  shown     323 

128.  A  color-mixture  instrument  for  studying  the  emotive  and  affective 

value  of  colors  and  color  phrases 324 

129.  Showing  the  relative  positions  of  the  colored  lamps  in  the  apparatus 

diagrammatically  shown  in  Fig.  128 325 

130.  Michrophotographs  of  white  cotton  and  silk  fabrics  against  a  black 

background 348 

131.  Spectral  reflection-factors  of  pigments .  352 

132.  Spectral  reflection-factors  of  pigments 353 

133.  Spectral  luminosities  of  pigments 360 

134.  Spectral  luminosities  of  pigments 361 

135.  A  study  of  a  pigment  (light  chrome  yellow) 362 

136.  Reflection-factors  of  pigments 368 

137.  Relative  reflection-factors  of  pigments 369 

138.  Influence  of  the  illuminant  on  the  appearance  of  a  pigment 370 

139.  Relation  between  spectral  transmission-factor  and  depth  or  concen- 

tration of  a  solution  of  methylengriin 381 

140.  Relation  between  spectral  luminosity  and  depth  or  concentration  of  a 

solution  of  rosazeine 382 

141.  Complete  relation  between  thickness,  wave-length,  and  transmission- 

factor  for  a  gold  ruby  glass 383 

142.  Spectral  transmission-factors  of  selenium  glasses 387 

143.  Spectral   transmission-factors   of   copper,   sulphur,   chromium,    and 

uranium  glasses 388 

144.  Spectral  transmission-factors  of  gold  glasses  and  combinations  with 

cobalt 389 

145.  Spectral  transmission-factors  of   carbon   glasses  and  combinations 

with  cobalt  glasses 390 

146.  Spectral  transmission-factors  of  cobalt  glasses 391 

147.  Spectral  transmission-factors  of  iron  and  of  manganese  glasses .    .    .  392 

148.  Relations  between  spectral  transmission-factor   and  thickness  of  a 

gold  glass  (23Au) 393 

149.  Relations  between  spectral  luminosity  and  thickness  of  a  gold  glass 

(23Au) 394 

150.  Test  of  the  relation  between  spectral  transmission-factor  and  thick- 

ness of  a  blue-green  glass \    .     395 


COLOR 

AND   ITS   APPLICATIONS 


CHAPTER   I 

LIGHT 

1.  The  word  Light  has   acquired   two   meanings; 
one  pertains  to  sensation  and  is  therefore  physiological 
and  psychological  in  character,  while  the  other  refers 
to  the  external  cause  of  the  sensation  and  is  there- 
fore physical  in  nature.     As  both  meanings  are  used 
in  the  study  of  color,  they  will  be  distinguished  wher- 
ever necessary;  for  example,  light  rays  when  imping- 
ing upon  the  retina  of  the  eye  produce  the  sensation 
of  light.     In  order  to  understand  the  phenomena  of 
color,   a   fair   knowledge    of   the    physical    nature   of 
light  must  first  be  acquired.     Unfortunately  the  field 
to  be  covered  in  this  book  is  too  extensive  to  permit 
of  a  detailed  treatment  of  this  interesting  subject; 
only   those    phenomena    will    be    discussed   that   are 
essential    to    an    understanding    of    the    subsequent 
chapters.     Those  wishing  to  pursue  this  line  of  study 
further   can   readily   do   so   by   consulting   the   many 
excellent  treatises  on  the  subject. 

2.  Wave    Theory. --The   passage   of  a   beam   of 
light  from  a  source  (flame,  sun,  etc.)  to  a  receiver 
(the  earth,  the  eye,  etc.)  involves  a  transfer  of  energy, 
and  the  question  arises  as  to  how  this  transfer  takes 
place.     All   around   us   in   Nature,   energy  is   contin- 
ually  being   transferred   from   one   place   to   another 
and  whenever  such  a  transfer  does  occur  something 
is   moving.     On   the   ocean,   for   example,   energy  is 


COLOR  AND  ITS  APPLICATIONS 


transferred  by  water  due  to  its  wave  motion.  Moun- 
tain streams  are  carrying  energy,  which  fortunately 
can  be  made  to  do  useful  work  by  means  of  a  water 
motor,  but  here  the  energy  is  transferred  by  the 
onward  flow  of  the  water.  In  air  the  same  two  meth- 
ods of  transferring  energy  are  found;  currents  in 
the  case  of  winds  and  waves  in  the  case  of  sounds. 
Solids  also  can  be  made  to  transmit  energy  in  these 
two  ways;  by  currents  as  in  the  sand  blast  and  by 
waves  as  in  the  case  of  sound  and  other  elastic  dis- 
turbances. Since  currents  and  waves  are  such  com- 
mon methods  of  transmitting  energy,  it  is  quite 
natural  that  they  should  be  called  upon  to  explain  the 
transfer  in  the  case  of  light.  Light  travels  in  straight 
lines,  casts  (comparatively)  sharp  shadows,  is  reflected 
from  a  smooth  surface  as  a  regular  succession  of 
rubber  balls  would  be  if  thrown  against  the  same 
surface,  and  in  many  other  ways  acts  much  like  a 
current  of  particles  would  act  if  projected  from  the 
source  of  light  at  a  high  velocity. 

There  is  one  phenomenon,  however,  that  cannot 
be  explained  by  the  assumption  of  a  current  of  par- 
ticles; under  certain  conditions  two  rays  of  light  of 
equal  intensity  can  be  sent  to  the  same  spot  in  such 
a  manner  that  the  spot  will  be  dark  and  not  twice  as 
bright  as  it  would  be  if  either  ray  were  present  alone. 
This  fact  is  explained  by  assuming  that  light  energy 
is  transmitted  in  the  form  of  wave  motion  for  it  is 
seen  that  if  two  equal  waves  are  made  to  pass  in 
the  same  direction  through  any  medium,  but  in  such 
a  manner  that  the  crests  of  one  wave  coincide  with 
the  troughs  of  the  other,  the  two  waves  will  annul 
each  other  everywhere,  there  will  be  no  resultant 
wave,  no  transfer  of  energy,  and  hence  no  light  at 
the  spot  in  question.  To  this  phenomenon  was  given 


LIGHT  3 

the  name  'interference,'  but  the  term  has  been 
extended  to  include  all  the  phenomena  that  may  take 
place  when  two  or  more  waves  travel  in  the  same  me- 
dium at  the  same  time.  The  foregoing  case  and  all 
others  in  which  there  is  destruction  of  motion  are 
now  grouped  under  the  term,  destructive  interference ; 
in  contradistinction  to  this,  there  is  constructive 
interference  wherever  the  motion  due  to  all  the 
waves  is  greater  than  that  due  to  one.  The  simplest 
case  of  the  latter  type  is  that  of  two  equal  wave  trains 
traveling  in  the  same .  direction  at  the  same  time  but 
in  such  a  manner  that  the  crests  of  one  coincide  with 
the  crests  of  the  other.  The  two  -waves  reinforce 
each  other  and  the  resultant  wave  has  twice  the  am- 
plitude of  the  original  waves.  Another  very  important 
special  case  of  interference  is  that  to  which  the  term 
'standing  wave1  or  'stationary  wave'  has  been  applied. 
This  occurs  whenever  two  equal  wave  trains  are 
passing  in  opposite  directions  through  the  same  me- 
dium at  the  same  time.  The  most  common  way  of 
obtaining  equal  waves  traveling  in  opposite  directions 
is  by  means  of  reflection  at  a  surface  perpendicular 
to  the  direction  in  which  one  train  is  traveling.  Stand- 
ing waves  can  be  readily  demonstrated  by  fastening 
one  end  of  a  long  rope  (preferably  so  that  it  hangs 
vertically)  and  by  shaking  the  other  end,  timing  the 
motion  of  the  hand  so  that  it  is  in  unison  with  the 
reflected  waves.  It  will  be  seen  that  some  points 
of  the  rope  remain  at  rest  and  others  swing  through 
a  large  amplitude.  The  points  at  rest  are  called 
nodes  and  the  part  of  the  string  between  the  nodes 
is  a  segment.  By  varying  the  speed  of  the  hand  or 
the  period  of  vibration  the  string  can  be  made  to 
vibrate  in  one,  two,  three,  or  more  segments. 

A  brief  consideration  of  wave  motion  in  general 


COLOR  AND   ITS  APPLICATIONS 


and  a  few  definitions  may  not  be  out  of  place  here. 
In  the  first  place,  it  is  evident  that  in  any  wave 
motion,  the  parts  of  the  medium  do  not  travel  as  far 
as  the  wave.  They  remain  each  in  its  own  region, 
each  causing  adjacent  parts  to  move  and  in  so  doing 
gives  up  to  the  adjacent  part  some  of  its  energy.  The 
motion  of  the  particles  differ  in  various  kinds  of 
waves.  7  In  waves  in  deep  water  each  drop  moves  in 
a  vertical  plane  in  a  circular  orbit.  In  waves  in  shal- 
low water  the  orbit  is  an  elongated  ellipse.  In 
media  transmitting  sound  wav^s  the  motion  of  each 
particle  is  to  and  fro  in  a  straight  line  in  the  direction 
in  which  the  wave  is  traveling.  For  all  waves  the 
wave-length  is  the  distance  between  any  two  suc- 
cessive particles  that  are  moving  through  the  same 
points  in  their  orbits  at  the  same  instant.  The  ampli- 
tude is  half  a  particle's  path  length  (the  diameter 
of  the  orbit).  The  period  is  the  time  taken  for  the 
wave  to  travel  one  wave-length.  The  frequency  is 
the  reciprocal  of  the  period  or  the  number  of  waves 
that  pass  a  given  point  in  a  unit  of  time.  If  two 
waves  are  'in  step'  so  that  a  crest  of  one  occurs  at 
the  same  time  and  place  as  a  crest  of  the  other, 
the  two  waves  are  said  to  be  in  phase.  If  a  wave  is 
confined  to  a  surface  such  as  that  due  to  a  pebble 
dropped  in  a  quiet  pond  of  water,  the  waves  will  be 
circular;  any  circle  is  a  wave  front,  and  the  direction 
in  which  the  wave  is  traveling  at  any  point  is  that  of 
the  radius  drawn  to  the  point  and  is  therefore  per- 
pendicular to  the  wave  front.  In  the  case  of  light 
under  ideal  conditions,  the  wave  will  spread  out  in 
all  directions  from  a  point,  so  that  the  wave  front 
will  be  spherical.  The  direction  of  propagation  will 
again  be  perpendicular  to  the  wave  front  along  the 
radii  of  the  sphere. 


LIGHT 


Light  energy  or  radiant  energy  passes  through  a 
vacuum.  The  phenomenon  of  interference  has  been 
explained  by  wave  motion.  Hence  it  is  assumed  that 
there  is  something  in  the  vacuum  that  can  move. 
This  something  is  called  the  ether  and  is  further 
assumed  to  penetrate  all  matter  so  that  light  waves 
always  are  ether  waves;  the  properties  of  the  waves 
may  change  as  the  matter  imbedded  in  the  ether  is 
changed,  but  it  is  the  ether  and  not  the  matter 
imbedded  in  the  ether  that  is  responsible  for  the 
propagation  of  the  light  waves.  Some  scientists, 
are  not  reconciled  to  this  view  but  fortunately  in  this 
treatise  we  need  not  enter  into  the  discussion. 

The  adoption  of  the  wave  theory  necessitates 
new  and  somewhat  elaborate  explanations  for  such 
simple  phenomena  as  the  rectilinear  propagation  of 
light;  these  have  been  made  by  the  aid  of  Huy- 
ghen's  principle,  which  states  that  each  point  on  a 
wave  front  may  be  regarded  as  a  new  source  of  dis- 
turbance, sending  out  spherical  waves,  and  that  at 
any  instant  the  new  wave  front  will  be  the  envel- 
ope of  all  of  these  secondary  wavelets.  By  the  aid 
of  this  principle  it  is  at  once  evident  that  a  light 
wave  in  going  through  a  wide  slit  will  pass  on  in 
such  a  manner  that  the  sides  of  the  slit  cast  a  rather 
sharp  shadow,  whereas  in  going  through  a  very  narrow 
slit,  comparable  in  width  with  a  wave-length  of  light, 
it  will  pass  on  and  spread  out  in  all  directions  thus 
*  turning  a  corner.'  This  phenomenon  has  been 
termed  diffraction.' 

It  is  helpful  to  visualize  light  waves  by  means 
of  the  water  wave  analogy  as  has  been  done  in  the 
foregoing,  but  it  is  well  to  guard  against  being  misled 
by  following  the  analogy  too  closely.  For  example 
water  waves  dimmish  in  amplitude  on  account  of 


6  COLOR  AND   ITS  APPLICATIONS 

dissipation  of  energy  through  molecular  friction.  In 
free  space  the  amplitude  of  light  waves  (which  is  a 
measure  of  their  intensity)  has  not  been  observed 
to  decrease;  in  other  words,  there  is  no  friction  in 
the  transmitting  medium. 

3.  Electro-magnetic  Theory.  —  At  first  it  does  not 
appear  that  there  is  any  relation  between  light  and 
electricity,  but  such  a  relation  was  predicted  by  Max- 
well in  about  1870.  This  theory  assumes  light  rays 
to  be  identical  with  the  electro-magnetic  disturbances 
which  are  radiated  from  a  body  in  which  electrical 
oscillations  are  taking  place.  Some  years  later  Hertz 
actually  produced  these  waves  and  by  this  discovery 
the  electro-magnetic  theory  expounded  by  Maxwell 
was  supplied  with  the  necessary  physical  foundation. 
In  enunciating  this  theory  it  is  customary  to  state 
that  the  oscillating  electrons  in  the  atoms  which 
constitute  a  body  send  forth  through  space  pulses 
of  electro-magnetic  energy.  The  electron  at  present 
is  supposed  to  be  an  atom  of  electricity.  These 
electric  waves  emanating  from  a  radiating  body 
whether  it  be  the  sun  or  a  red-hot  poker  have  many 
properties  depending  upon  their  wave-length.  Al- 
though all  travel  at  the  same  velocity,  about  3  x  10  10 
centimeters  (186000  miles)  per  second,  in  free  space, 
they  differ  somewhat  in  velocity  in  the  ordinary  trans- 
parent media.  In  glass,  for  instance,  violet  rays  travel 
less  rapidly  than  the  red  rays.  All  these  rays  represent 
energy  and  therefore  regardless  of  wave-length  have 
the  property  of  producing  heat  when  absorbed.  Some- 
times the  energy  is  not  wholly  converted  into  heat 
but  enters  into  chemical  reactions  or  is  converted 
into  electricity  or  radiant  energy  of  other  wave-lengths 
than  those  of  the  absorbed  rays.  Some  of  the  rays, 
especially  those  of  shorter  wave-length  than  the  visible 


LIGHT 


rays,  are  very  active  chemically,  affecting  a  photo- 
graphic emulsion,  destroying  bacteria  and  animal, 
tissues  such  as  the  outer  membrane  of  the  eye 
and  causing  sunburn;  they  are  also  largely  respon- 
sible for  exciting  phosphorescence  and  fluorescence. 
Other  rays  have  varying  effects  on  organisms  and 
play  a  more  or  less  important  part  in  the  growth  of 
plants.  Rays  within  a  certain  range  of  wave-lengths, 
separately  and  in  groups,  produce  the  sensation  of 
light  and  color.  In  other  words  the  retina  of  the 
eye  can  be  likened  to  a  receiving  station  in  wireless 
telegraphy  which  is  *  tuned'  to  respond  to  electro- 
magnetic rays  of  a  certain  (limited)  range  of  wave- 
lengths. 

4.  Radiation  and  Light  Sensation.  —  There  are 
many  ways  of  decomposing  radiation  into  its  various 
component  rays.  The  rainbow  is  the  result  of  one 
of_Nature's  means  of  dispersing  the  radiation  from 
the  sun  into  rays  of  various  wave-lengths.  The  eye 
sees  in  the  rainbow  many  colors,  the  most  conspicu- 
ous being  violet,  blue,  green,  yellow,  orange  and  red. 
These  are  seen  to  be  of  different  brightnesses.  If 
the  retina  were  sensitive  to  rays  of  an  infinite  range 
of  wave-lengths  the  rainbow  would  appear  much  wider 
than  it  does.  That  is,  colors  whose  appearance 
can  not  be  imagined,  would  be  seen  in  the  now  in- 
visible regions  beyond  the  violet  and  red,  because 
energy  corresponding  to  those  wave-lengths  is  present 
in  sunlight. 

The  distribution  of  the  energy  among  the  different 
wave-lengths  radiated  by  a  hot  solid  is  shown  in 
Fig.  1.  This  curve  is  technically  known  as  a  radia- 
tion curve  and  shows  that  energy  of  a  great  range 
of  wave-lengths  is  present.  Such  a  continuous  spec- 
trum is  characteristic  of  the  radiation  from  solid 


8 


COLOR  AND  ITS  APPLICATIONS 


bodies.  On  the  basis  of  the  electro-magnetic  theory 
of  light,  it  may  seem  strange  that  rays  of  all  wave- 
lengths are  produced  by  the  vibrating  electrons. 
This  may  be  pictured  sufficiently  well  for  the  present 
purpose  by  assuming  that  in  a  solid  body  there  is 
considerable  damping  of  the  vibrations  and  that  other 
influences  are  present  which  result  in  the  emission 


V          R  WAVE  LENGTH 

Fig.  1.  —  Radiation  curve  of  an  incandescent  solid. 

of  no  characteristic  single  ray  or  series  of  rays  but, 
instead,  of  rays  of  all  wave-lengths.  The  height  of 
the  curve  at  any  point  above  the  axis  of  abscissae  or 
base  line  represents  the  relative  amount  of  energy  of 
that  particular  wave-length  present  in  the  total  radia- 
tion. It  will  be  noted  that  the  amounts  of  energy  of 
various  wave-lengths  are  by  no  means  of  the  same 
value.  This  characteristic  of  illuminants  is  of  great 
importance  in  a  study  of  colors  as  will  become  evident 
later.  The  region  to  which  the  eye  is  *  tuned,'  the 
visible  spectrum,  lies  between  V  (violet)  and  R  (red) 
which  is  exaggerated  in  relative  extent  for  the  sake 
of  clearness. 

The  relation  between  radiation  and  light  sensa- 
tion is  not  simple.  The  ability  of  the  various  rays 
to  produce  light  sensation  is  shown  roughly  by  the 
dotted  curve  in  Fig.  1.  The  maximum  light  sensa- 
tion is  produced  by  rays  in  the  middle  of  the  visible 
spectrum,  namely  by  those  giving  rise  to  a  yellow- 


LIGHT.  9 


green  sensation.  Beyond  the  limits  of  the  visible 
spectrum,  V  and  #,  it  is  obvious  that  an  infinite 
amount  of  electro-magnetic  energy  causes  no  sensa- 
tion of  light.  The  total  range  of  wave-lengths  might 
be  called  the  energy  spectrum  of  this  particular  radi- 
ator. It  is  obvious  that  the  greater  the  percentage 
of  the  total  radiant  energy  confined  to  the  visible 
spectrum,  the  greater  is  the  'luminous  efficiency'  of  the 
radiating  body.  In  the  production  of  light  the  total  *] 
range  of  wave-lengths  is  of  interest,  but  in  the  con- 
sideration of  Color,  interest  is  very  largely  confined 
to  the  visible  spectrum. 

As  the  temperature  of  an  incandescent  body  is 
increased,  the  energy  of  the  shorter  wave-lengths 
increases  more  rapidly  than  the  energy  of  the  longer 
wave-lengths.  Considering  the  visible  spectrum,  the 
violet  and  the  adjacent  rays  increase  in  intensity 
more  rapidly  with  increase  of  temperature  than  the 
red  and  its  adjacent  rays  thus  causing  the  light 
emitted  by  an  incandescent  filament,  for  instance,  to 
become  bluer  as  its  temperature  is  increased.  Here 
it  is  well  to  note  that  when  the  various  visible  rays 
are  permitted  to  impinge  separately  upon  the  retina 
each  produces  its  own  color  sensation  but  when  all 
the  visible  rays  simultaneously  stimulate  the  retina, 
as  in  the  ordinary  viewing  of  colorless  objects,  an 
integral  sensation  is  produced.  In  the  case  of  most 
common  illuminants  the  integral  sensation  is  an 
unsaturated  yellow,  that  is,  a  yellowish  white,  while 
the  combined  sensations  aroused  by  average  day- 
light produce  the  integral  sensation  of  white  light. 

In  the  discussion  of  Fig.  1  it  has  been  seen  that 
energies  in  the  various  wave-lengths  .differ  in  light- 
producing  effects.  To  this  must  be  added  the  fact 
that  the  light-producing  effect  varies  with  the  inten- 


10 


COLOR  AND   ITS  APPLICATIONS 


sity.  In  Fig.  2,  A  represents  the  light  sensation  pro- 
duced in  the  author's  eye  by  equal  amounts  of  energy 
of  various  wave-lengths  as  measured  with  a  direct 
comparison  or  equality-of-brightness  photometer.  The 
photometric  field  was  a  circle  whose  diameter  sub- 


100 

so 

80 

I  70 
o 

I  60 
3  50 


H  40 


\ 


\ 


\ 


0.40        044  045          052  Q56  Q60          064          0.68 

M,  WAVE  LENGTH 

Fig.  2.  —  Showing  the  relation  between  radiant  energy  and  light 
sensation. 

tended  an  angle  of  about  four  degrees  at  the  eye,  and 
whose  brightness  was  equivalent  to  that  of  a  white 
surface  illuminated  to  an  intensity  of  20  meter  candles ; 
(a  meter  candle  is  the  illumination  received  on  a 
surface  everywhere  one  meter  distant  from  a  source 
of  one  candle,  the  surface  being  perpendicular  to 
the  straight  line  from  it  to  the  light  source).  On 
decreasing  the  intensity  and  therefore  the  brightness 
of  the  photometer  field  to  about  one  two-hundredth 
of  its  original  value  or  to  an  equivalent  of  0.1  meter 
candle  on  the  foregoing  basis,  the  relation  between 
light  and  radiation  become  as  shown  in  curve  B. 
It  will  be  noted  that  the  maximum  of  the  luminosity 
curve  (as  it  is  called)  has  shifted  toward  the  shorter 


LIGHT  11 


wave-lengths.  In  other  words  at  the  low  illumina- 
tion the  light-producing  effect  of  visible  rays  of  the 
shorter  wave-lengths  has  not  decreased  as  much  as 
that  of  the  longer  wave-lengths.  To  illustrate  by 
a  simple  experiment,  suppose  a  red  and  a  blue  sur- 
face appear  of  equal  brightness  at  a  high  illumina- 
tion. If  the  intensity  of  illumination  is  reduced  to 
a  very  low  value,  the  blue  surface  will  appear  much 
brighter  than  the  red  one.  To  further  complicate 
matters  it  is  found  that  even  normal  eyes  differ  some- 
what in  spectral  sensibility  for  experiment  shows  that 
the  luminosity  curves  for  various  normal  eyes  do  not 
exactly  coincide  (see  #  56).  Curve  C  is  plotted  from 
Koenig's  1  data,  obtained  at  a  very  low  illumination, 
practically  at  the  threshold  of  vision.  This  phenome- 
non of  shifting  spectral  sensibility  which  was  discov- 
ered by  Purkinje,  and  which  bears  his  name,  will  be 
discussed  in  later  chapters,  and  the  quantitative  rela- 
tion between  radiation  and  light  sensation  will  be  fur- 
ther treated  in  the  chapter  on  color  photometry. 

5.  Temperature  and  Radiation.  —  As  already 
stated  the  effect  of  raising  the  temperature  of  a 
heated  solid  body  is  to  increase  the  luminous  effi- 
ciency and  also  the  relative  amount  of  energy  in 
the  rays  of  shorter  wave-lengths.  These  effects  are 
shown  diagrammatically  for  a  solid  body  in  Fig.  3. 
The  numbers  on  the  curves  indicate  the  absolute 
black-body  temperatures.  The  wave-length  is  given  in 
terms  of  ten-thousandths  of  a  centimeter,  this  unit 
being  usually  expressed  by  the  Greek  letter,  /x.  The 
rays  to  which  the  eye  is  sensitive  lie  between  V  and  /?, 
respectively  about  0.4/z  and  0.7^.  The  eye  in  reality 
is  sensitive  somewhat  beyond  these  wave-lengths  but 
for  practical  purposes  the  amount  of  light  sensa- 
tion produced  by  rays  beyond  these  limits  is  usually 


12 


COLOR  AND   ITS  APPLICATIONS 


negligible.  It  is  seen  that  as  the  temperature  rises 
the  maximum  of  the  radiation  curve  shifts  toward 
the  shorter  wave-lengths.  The  maximum  of  the  radi- 
ation curve  of  sunlight  lies  in  the  visible  region. 
This  has  brought  forth  the  suggestion  that  the  eye 

80 


234 

WAVE  LENGTH 

Fig.  3.  —  Showing  the  effect  of  temperature  on  the  radiation 
from  an  incandescent  solid  (black-body). 

in  its  process  of  evolution  has  become  most  sensitive 
to  the  rays  of  such  wave-length  as  are  a  maximum 
in  the  radiation  from  the  sun.  As  the  maximum  of 
the  radiation  curve  shifts  toward  the  shorter  wave- 
lengths, it  is  seen  that  a  relatively  greater  proportion 
of  the  total  energy  is  found  in  the  visible  region 
between  V  and  R  which  accounts  for  the  increase,  in 
luminous  efficiency.  One  of  the  tendencies  in  light 
production  is  toward  the  development  of  materials 
and  methods  which  will  enable  the  light  source  to  be 
operated  at  higher  temperatures  in  order  to  appease 
the  ever-present  demand  for  higher  luminous  effi- 
ciencies. It  is  evident  that  the  ideal  light  source 
emitting  a  continuous  spectrum  would  be  one  that 
radiated  no  energy  beyond  V  and  R.  The  area 
under  each  curve  is  proportional  to  the  total  amount 
of  energy  emitted  by  the  radiator  at  a  certain  tern- 


LIGHT  13 

perature  and  the  ratio  of  the  area  under  that  part 
of  any  curve  included  between  V  and  R  is  propor- 
tional to  the  energy  that  can  effect  the  eye.  The 
ratio  of  the  latter  area  to  the  former  (for  the  same 
curve)  is  called  the  *  radiant  efficiency'  of  the  radiator 
as  a  light  source.  To  make  the  idea  of  radiant  effi- 
ciency of  practical  value  it  must  be  combined  with  the 
relations  between  luminous  sensation  and  radiation. 

6.  Spectra  of  Illuminants.  —  The  spectral  distribu- 
tion of  energy  in  the  radiation  from  different  illumi- 
nants  is  of  great  importance  in  the  consideration  of 
color  owing  to  the  fact  that  the  appearance  of  the 
colored  objects  depends  upon  the  spectral  character 
of  the  illuminant  under  which  they  are  viewed.  The 
variation  in  the  spectral  character  of  illuminants  is 
due  to  the  temperature  and  composition  of  the  radi- 
ating body  and  also  to  the  state  in  which  it  exists 
when  radiating  luminous  energy. 

A  simple  means  of  producing  light  is  that  of 
heating  a  solid  conductor  by  passing  an  electric 
current  through  it.  At  first  it  will  emit  invisible 
radiant  energy  known  as  infra-red  rays.  As  the 
temperature  is  raised  it  will  finally  become  luminous, 
at  first  appearing  a  dull  red.  This  is  evident  from  an 
inspection  of  Fig.  3.  If  these  light  rays  be  studied 
by  means  of  a  spectroscope  which  disperses  the 
radiation  into  its  component  rays,  it  will  be  found 
that  deep  red  rays  are  the  only  visible  rays  present 
in  appreciable  amounts.  As  the  temperature  is  in- 
creased the  appearance  of  the  body  passes  from  red 
to  orange,  then  to  yellow  and  so  on.  If  the  body 
were  sufficiently  refractory  to  withstand  higher  tem- 
peratures and  remain  in  solid  form,  at  a  certain  tem- 
perature it  would  appear  white  and  with  increasing 
temperature  would  assume  a  bluish  white  appearance. 


14  COLOR  AND   ITS  APPLICATIONS 

The  latter  temperatures  have  never  been  reached  in 
the  production  of  artificial  light.  Notwithstanding 
the  fact  that  all  solids  produce  .a  continuous  spectrum 
and  obey  the  general  laws  mentioned,  it  does  not 
follow  that  they  all  emit  the  same  amounts  of  light 
per  unit  area  at  equal  temperatures.  Kirchhoff  has 
shown  by  the  theory  of  exchanges  that  the  emissive 
and  absorptive  powers  of  all  bodies  at  the  same  tem- 
perature for  rays  of  a  particular  wave-length  are  pro- 
portional to  each  other  when  the  radiation  is  a  pure 
temperature  effect.  For  a  particular  kind  of  radiator 
called  a  black  body  or  a  full  radiator,  the  relation 
between  emission  £,  the  wave-length,  X,  and  the 
absolute  temperature  J,  has  been  deduced  theoretic- 
ally. The  black  body  is  defined  as  a  body  that 
will  absorb  all  radiation  incident  upon  it  and  reflect 
none.  When  it  radiates  it  emits  in  each  wave-length 
more  energy  than  any  other  body  at  the  same  tem- 
perature. The  nearest  approach  to  such  an  ideal 
radiator  is  a  hollow  space  enclosed  by  emitting  walls 
of  uniform  temperature  provided  with  a  small  open- 
ing through  which  the  radiation  can  escape.  The 
laws  deduced  theoretically  by  various  investigators 
do  not  agree  entirely.  The  one  that  best  fits  exper- 
imental data  is  Planck's  law  given  in 

Ex=  C^-'(e*T  ..I)-'  (1) 

Another  law  known  as  the  Wien-Paschen  law  found 
to  hold  for  the  short-wave  region  of  the  visible  spec- 
trum is  shown  in  equation  (2). 

£x=  CU-'e"^  (2) 

In  the  foregoing  equations  Ci  and  c2  are  constants. 
The  values  for  these  differ  somewhat  as  determined 
by  various  investigators. 


LIGHT  15 

A  simple  relation  between  the  wave-length  of  the 
maximum  of  the  radiation  curve  and  the  absolute 
temperature  is  derived  from  (2)  and  is  expressed  in 
equation  (3). 

XmT  =  constant  (3) 

Another  interesting  relation  known  as  the  Stefan- 
Bbltzmann  law  connects  the  total  radiation,  E,  from 
a  unit  area  of  the  radiator  with  the  absolute  temper- 
ature, T,  and  is  expressed  in  equation  (4). 

E=  C714  (4) 

These  laws  are  of  chief  importance  in  the  theory  of 
radiation  but  are  given  here  as  a  matter  of  reference. 
A  gaseous  body  is  found  to  emit  only  certain 
definite  rays  and  the  spectrum  is  said  to  be  a  line 
spectrum.  Sometimes  the  various  lines  (which  are 
in  reality  the  images  of  the  slit  of  the  spectroscope, 
(8)  are  found  to  be  crowded  together  in  such  a 
manner  as  to  give  to  the  spectrum  a  fluted  appear- 
ance. Such  a  spectrum  is  called  a  banded  or  fluted 
spectrum.  A  further  striking  fact  is  the  constancy 
of  the  appearance  of  the  spectra  emitted  by  elements 
in  gaseous  form.  For  instance  the  spectrum  of 
sodium  is  always  recognized  by  the  position  of  the 
emitted  rays  in  the  spectrum  —  that  is,  by  their 
wave-length.  The  visible  spectrum  of  sodium  con- 
sists of  a  double  line  (0.5890^  and  0.5896/0  and 
whenever  this  double  line  is  found  in  a  spectrum  it 
is  certain  that  sodium  is  present  in  the  radiating 
substance.  This  constancy  of  the  spectra  of  the 
elements  forms  the  basis  of  spectrum  analysis  by 
means  of  which  traces  of  elements  far  too  small  to 
be  weighed  by  the  most  sensitive  balance  are  readily 
detected.  By  means  of  the  spectroscope  helium  was 
discovered  on  the  sun  before  it  was  distinctly  isolated 


16  COLOR  AND   ITS  APPLICATIONS 

by  scientists  on  earth.  The  vacuum  tube,  the  arc, 
the  electric  spark,  and  the  flame  are  used  in  studying 
the  spectra  of  elements  and  compounds. 

Sometimes  both  a  line  and  a  continuous  spectrum 
are  emitted  by  an  illuminant.  Such  a  case  is  found 
in  the  ordinary  carbon  electric  arc.  The  crater  of 
the  arc  being  an  incandescent  solid,  emits  all  visible 
rays  while  the  incandescent  gas  of  the  arc  between 
the  electrodes  emits  a  line  spectrum  the  appearance 
of  which  depends  upon  the  surrounding  medium  and 
the  composition  of  the  carbons.  In  Fig.  4  are  shown 
several  representative  spectra  photographed  by  means 
of  a  grating  spectrograph  on  a  Cramer  spectrum  plate 
which  is  sensitive  in  varying  degrees  to  all  the  vis- 
ible rays.  This  particular  brand  of  photographic 
plate  is  relatively  less  sensitive  to  blue-green  rays 
so  that  on  viewing  the  spectrograms  the  energy  in 
this  region  appears  to  be  less  prominent  than  it 
really  is.  It  is  seen  that  the  two  gases,  mercury 
and  helium,  emit  line  spectra.  The  arcs  emit  both 
continuous  and  line  spectra,  the  latter  as  indicated 
above  being  emitted  by  the  vapor  in  the  arc  itself. 
The  relative  prominence  of  the  line  spectra  depends 
upon  the  relative  intensities  of  the  radiation  from 
the  arc  as  compared  to  that  from  the  solid  electrodes. 
For  instance  the  line  spectrum  is  much  more  prom- 
inent in  the  yellow  flame  arc  than  in  the  ordinary 
carbon  arc  and  as  is  well  known  the  arc  vapor  con- 
tributes a  much  greater  proportion  of  the  light  in  the 
former  than  in  the  latter  illuminant.  The  line  spec- 
trum of  a  carbon  arc  is  subject  to  momentary  changes 
both  in  character  and  intensity  due  to  impurities 
and  also  to  irregularities  in  the  amounts  of  the  chem- 
icals with  which  the  carbons  are  impregnated.  The 
injection  of  various  chemicals  into  the  arc  as  sug- 


Visible  Spectrum 


a.  Mercury  Arc 


b.  Helium 


c.  Iron  Arc 


d.  Ye/ low  Flame  Arc 


e.  Carbon  Arc 


f.  Carbon  Arc 


g.  Carbon  Arc 


h.  Tungsten  Incandescent  Lamp 


i.  Skylight 


J.  Skylight 


V  B  0  Y  O  R 
Visible  Spectrum 


Fig.  4. — Representative  spectra. 


18  COLOR  AND  ITS  APPLICATIONS 

gested  by  the  heating  of  metallic  salts  in  a  Bunsen 
flame,  affords  a  means  of  varying  the  color  or  spectral 
character  of  the  light  from  the  carbon  arc  lamps. 
Spectra  /  and  g  were  obtained  from  the  same  carbon 
arc  within  a  period  of  a  few  seconds.  The  tungsten 
filament  is  seen  to  emit  a  continuous  spectrum,  ft, 
the  dark  band  being  due  to  the  low  sensibility  of  the 
photographic  plate  to  blue-green  rays.  Two  spectro- 
grams of  light  from  the  sky  are  shown  in  i  and  j 
in  an  effort  to  bring  out  the  dark  lines  which  cross 
the  spectrum. 

The  solar  spectrum  is  of  special  interest.  As 
already  indicated  a  photograph  of  the  spectrum  of 
sunlight  made  with  a  narrow  slit,  shows  practically 
a  continuous  band  crossed  by  many  fine  dark  lines 
(see  Plate  I).  These  lines  were  discovered  by 
Wollaston  in  1802  but  were  later  studied  with  better 
instruments  by  Fraunhofer  in  1814  who  found  several 
hundreds  of  them.  These  dark  lines  in  the  position 
of  various  colors  show  the  absence  of  the  corre- 
sponding images  of  the  slit  of  the  instrument  and 
therefore  the  absence  of  these  rays  in  sunlight. 
Their  absence  is  attributed  to  absorption  by  vapor 
chiefly  in  the  solar  atmosphere.  Luminous  gases  or 
vapors,  as  has  already  been  indicated,  emit  only  a 
limited  number  of  rays,  their  spectra  being  dis- 
continuous in  appearance.  These  vapors  when  lumi- 
nous are  usually  opaque  to  the  particular  rays  which 
they  emit  and  therefore  the  light  from  the  sun  is 
robbed  of  some  of  the  rays  in  passing  through  its 
atmosphere.  The  Fraunhofer  lines  are  often  used  as 
reference  points  in  examining  spectra,  although 
electric  discharges  through  gases  in  vacuum  tubes  and 
the  heating  of  salts  in  a  gas  flame  furnish  convenient 
means  of  identification  or  reference  spectra  in  the 


LIGHT 


19 


experimental  laboratory.     The  chief  Fraunhofer  lines 
are  given  in  Table  I. 

TABLE  I 
Principal  Fraunhofer  Lines 


Line 

Wave-length 

Color 

Source 

A 

0.7594M 

Red 

Oxygen  in  atmosphere 

a 

.7185 

K 

Water  vapor  in  atmosphere 

B 

.6867 

u 

Oxygen  in  atmosphere 

C 

.6563 

K 

Hydrogen,    sun 

Di 

.5896 

Yellow 

Sodium,           " 

D2 

.5890 

14 

<«                « 

E 

.5270 

Green 

Calcium,         " 

b! 

.5184 

« 

Magnesium,   " 

b2 

.5173 

<c 

<{            « 

b4 

.5168 

« 

«            « 

F 

.4861 

Blue 

Hydrogen,      " 

G 

.4308 

Violet 

Calcium,         " 

H 

.3969 

ii 

<{                <( 

K 

.3934 

« 

«                « 

Other  convenient  lines  obtained  by  heating  salts 
in  a  Bunsen  flame  are — potassium  red,  0.7699  and 
0.7665  fjiy  lithium  red,  0.6708;  sodium  yellow,  0.5896 
and  0.5890;  thallium  green,  0.5351;  magnesium 
green,  0.5184;  strontium  blue,  0.4607.  The  mer- 
cury arc  gives  a  double  yellow  line,  0.5790  and  0.5764; 
a  bright  green  line,  0.5461;  a  faint  blue-green  line, 
0.4916;  an  intense  blue  line,  0.4358;  and  deep  violet 
lines,  0.4078  and  0.4047.  Hydrogen  at  a  fairly  low 
pressure  in  a  glass  tube  through  which  an  electric 
discharge  is  passed  yields  a  red  line,  0.6563,  blue- 
green,  0.4861,  and  blue  0.4341.  The  helium  spec- 
trum obtained  from  a  tube  such  as  the  foregoing  is 
rich  in  useful  lines  throughout  the  spectrum,  yielding 
red  lines  0.7282,  0.7065,  0.6678;  yellow  0.5876; 
green  lines  0.5048,  0.5016,  0.4922;  blue  lines  0.4713, 


20 


COLOR  AND   ITS  APPLICATIONS 


0.4472,  0.4388;  and  violet  lines  0.4026,  0.3888.  Iron, 
copper,  zinc,  and  cadmium  when  used  as  the  ter- 
minals of  an  electric  spark  yield  many  useful  lines, 
many  of  which  are  in  the  ultra-violet.  The  iron  arc 
and  the  quartz  mercury  arc  are  particularly  useful 
for  exploring  the  ultra-violet  region.  Three  useful 
cadmium  lines  are  red  0.6438,  green  0.5086,  and  blue 
0.4800. 


280 


LENGTH 

Fig.  6.  —  Distribution  of  energy  in  the  visible  spectra  of 
various  illuminants. 

In  Fig.  5  and  Table  II  are  shown  the  spectral  dis- 
tributions of  energy  in  the  visible  region  for  various 
illuminants.  These  data  have  been  obtained  in  the 
Nela  Research  Laboratory  chiefly  by  Hyde,2  Ives,3 
Cady  and  the  author.4  As  is  evident  at  a  glance  these 


LIGHT 


21 


TABLE   II 

• 

Relative  Distribution  of  Energy  in  the  Visible  Spectra  of  Common  Uluminants 


A 

B 

C 

D 

E 

F 

G 

H 

I 

0 

d 

13  2-     tS 

Q<         "Jw 

to 

•si 

M 

~!l 

11  j| 

§     1 

? 

I 

:ii 

P. 

O     2t 

"-"  a>  c  w 

MOJ^   P. 

& 

E 

1> 

"o  «  w 

8^ 

d 

g»*S 

^g«6g 

0 

•S3 

1 

3 

i 

Hefner 

Carbon 
incande 
lamp  3.: 

1 

Q 

<J 

.2  v  *?  a 

iKl 

Z*22 

£  v  x  ** 

11  fl 

llSs 

d 
Q 

Welsba< 
man! 

0.41M 

72. 

j 
177. 

1.9 

4. 

5.5 

16.5 

.43 

79. 

185. 

3.5 

7. 

9.6 

22.5 

21.8 

.45 

84.3 

187. 

6. 

12. 

15. 

16.7 

30. 

29. 

17.5 

.47 

91. 

180. 

10.5 

18. 

21.9 

23.5 

38. 

37. 

26.4 

.49 

92.5 

162. 

16.3 

25.5 

30.3 

32.7 

47. 

45.5 

38.3 

.51 

96. 

146. 

25.5 

34.5 

40. 

42.6 

56.5 

55. 

51. 

.53 

98. 

132. 

37.5 

47. 

52. 

54.9 

67. 

65.5 

64. 

.55 

99. 

120. 

53.2 

62. 

66.5 

68.6 

78. 

76. 

78. 

.57 

100. 

108. 

74.5 

79. 

82. 

83.4 

88. 

88. 

90. 

.59 

100. 

100. 

100. 

100. 

100. 

100. 

100. 

100. 

100. 

.61 

100. 

93. 

130. 

123. 

118. 

117. 

111. 

113.5 

107. 

.63 

98.5 

87. 

168. 

148. 

139. 

136. 

121.5 

127. 

111. 

.65 

97.1 

82. 

210. 

176. 

160. 

157. 

131. 

142. 

114. 

.67 

95.5 

77. 

260. 

204. 

182. 

179. 

140. 

156. 

119. 

.69 

93.5 

72.5 

320. 

234. 

205. 

202. 

147.5 

170. 

120. 

curves  are  plotted  in  such  a  manner  that  the  relative 
energy  of  wave-length  0.59  ^  equals  100.  This  method 
of  plotting  gives  the  relative  distribution  of  energy 
for  approximately  the  same  amounts  of  total  light 
sensation.  Reference  to  the  spectral  distribution 
of  energy  in  the  radiation  from  the  two  tungsten 
lamps,  operating  at  7.9  and  22  lumens  per  watt 
respectively,  shows  the  effect  of  increasing  temper- 
ature upon  the  relative  amounts  of  rays  of  shorter 
wave-length  emitted.  See  Table  XXIII. 


22  COLOR  AND   ITS  APPLICATIONS 

REFERENCES   CHAPTER  I 

1.  Ges.  Abhandlungen  zur  Physiol.  Optik.  Leipzig,  1903,  p.  144  et 

seq. 

2.  Jour.  Franklin  Inst.,  1910,  p.  439. 

3.  Trans.  I.E.S.,  1910,  p.  189. 

4.  Elec.  World,  Sept.  19,  1914;  Trans.  I.E.S.,  1914,  p.  839. 

OTHER  REFERENCES 

Theory  of  Optics.    Preston. 

Physical  Optics.    R.  W.  Wood. 

Outline  of  Applied  Optics.     P.  G.  Nutting. 

Lectures  on  Illuminating  Engineering.     Johns  Hopkins  University. 


CHAPTER   II 
THE  PRODUCTION   OF   COLOR 

7.  Colors  and  color  phenomena  are  encountered 
in  nearly  every  human  activity.     In  fact  they  are  so 
ever-present   that   they   have   become   common-place 
to    the    average    man    excepting    in    those    instances 
where  they  enter  actively  into  his  work.     That  there 
is  an  explanation  for  every  case  of  color  production 
is   inevitable,    however,    there    are    some    color   phe- 
nomena  as   yet  unexplained   satisfactorily.     Color  is 
produced  in  many  ways.     It  is  intimately  associated 
with  light  as  has   already  been   seen.     When  color 
is  produced  it  is  usually  the  result  either  of  a  sub- 
traction of  some  of   the  visible  rays  from  the  total 
radiation  emitted  by  light  source  or  of  the  dispersion 
of  the  visible  radiation  into  its  component  parts. 

8.  Refraction.  —  Sir   Isaac    Newton   in    1666    dis- 
covered that  sunlight  consisted  of  many  rays  each  of 
which  when  permitted  to  impinge  separately  upon  the 
retina    produced    the    sensation    of    a    distinct    color. 
He  accomplished 

this  by  prismatic 
dispersion  and 
proved  that  no 
further  change 
resulted  from 
subsequent  dis- 
persion. Newton's  experiment,  which  is  shown  dia- 
grammatically  in  Fig.  6,  was  performed  approximately 
in  the  following  manner.  A  prism  was  set  up  in  a 

23 


Fig.  6.  —  Newton's  experiment. 


24  COLOR  AND  ITS  APPLICATIONS 

darkened  room  and  sunlight  admitted  through  a  small 
hole  in  the  window  shade.  This  provided  him  with  a 
parallel  beam  of  light.  After  the  beam  traversed  the 
prism  he  found  that  there  was  no  longer  an  image  of 
the  sun,  but  a  colored  band  similar  to  the  rainbow, 
made  up  in  reality  of  an  infinite  number  of  colored 
images  of  the  slit  overlapping  each  other.  On  passing 
a  narrow  portion  of  this  colored  band  through  a  hole  in 
another  screen  and  permitting  this  to  traverse  another 
prism,  he  found  no  further  change  in  color,  thus  prov- 
ing that  monochromatic  light  can  not  be  further  de- 
composed. The  principles  involved  in  this  experiment 
are  used  today  in  a  great  deal  of  spectroscopic  work. 

Let  us  examine  some  of  the  characteristics  of  the 
spectrum  somewhat  more  in  detail.  It  will  be  found 
that  an  increase  in  the  width  of  the  slit  produces  an 
increase  in  the  brightness  of  the  spectrum  but  owing 
to  the  facts  that  the  spectrum  consists  of  overlapping 
images  of  the  slit  and  that  each  image  of  a  broad 
\j  3  5  Y  si**  is  wider  than  an  image  of 

a  narrow  slit,  it  is  evident 
that  there  will  be  more  over- 
lapping of  the  images  pro- 
duced with  a  wide  slit.  The 
smaller  the  amount  of  over- 
lapping the  purer  the  spectrum 
will  be.  The  difference  in  the 
appearance  of  the  spectrum 

the  grating    spectrum"  of    the    of    a    mercury    arc    due    to    a 

mercury  arc.  change  from  a  narrow  rectan- 

gular slit  to  a  wide  circular  opening  is  shown  in  Fig.  7; 
both  spectra  were  photographed  on  the  same  grating 
spectrograph.  The  material  of  which  a  prism  is  made 
also  has  a  marked  effect  on  the  appearance  of  the  spec- 
trum. When  a  ray  of  light  passes  through  a  prism  it  is 


THE  PRODUCTION   OF  COLOR 


25 


of  course  bent  out  of  its  original  direction,  the  amount 
of  bending  or  the  angle  of  deviation  depending  upon 
the  angle  of  the  prism  and  the  index  of  refraction  for 
the  particular  wave-length  of  which  the  ray  consists. 
If  for  a  given  prism  the  angle  of  deviation  remained  the 
same  while  the  wave-length  of  the  light  was  changed, 
then  the  index  of  refraction  would  also  remain  con- 
stant, and  if  wave-lengths  were  plotted  against  either 


1.72 
1.70 
1.68 
1.66 
1.64 
1.62 
1.60 
1.56 
1.56 
1.54 
1.52 


0.74 


0.82 


QM        0.42         0.50          0.58          0.66' 
JA,  WAVE  LENOTH 

Fig.  8.  —  Dispersion  curves  of  various  optical  media. 

angles  of  deviation  or  indices  of  refraction  the  disper- 
sion curve  would  be  a  straight  line.  If  such  a  substance 
actually  existed,  it  would  be  possible  to  determine 
wave-lengths  in  its  spectrum  by  the  use  of  an  ordinary 
graduated  scale.  However,  such  is  not  the  case  and 
further,  the  dispersion  curves  of  various  prisms  differ 
considerably.  The  dispersion  curves  of  quartz  and 
three  kinds  of  glass  are  presented  in  Fig.  8.  It  is 
evident  from  an  inspection  of  these  curves  that  the 
different  wave-lengths  are  more  closely  crowded  to- 
gether in  the  red  or  long  wave-length  end  of  the 


26 


COLOR  AND  ITS  APPLICATIONS 


spectrum  than  in  the  other.  This  is  shown  in  Plate 
I  (Frontispiece)  where  a  prismatic  spectrum  is  com- 
pared with  a  normal  (grating)  spectrum  in  which  equal 
distances  represent  equal  wave-length  intervals. 

For  the  study  of  the  visible  rays  transparent  glass 
prisms  are  satisfactory  but  for  the  study  of  the  invis- 
ible rays  other  media  must  be  used  because  glass  is 
not  sufficiently  transparent  for  very  short  or  very 
long  waves.  Hence  investigations  of  the  ultra-violet 
region  are  made  with  optical  systems  of  quartz  and 
those  of  the  infra-red  with  fluorite  or  rock  salt.  Ex- 
tremely long  waves  such  as  are  used  in  wireless 
telegraphy  are  studied  by  means  of  huge  prisms  of 
pitch  or  paraffin. 

9.  Diffraction.  —  Another  common  device  employed 
in  optical  instruments  for  decomposing  visible  radia- 
tion into  its  components  is  the 
so-called  diffraction  grating.  A 
grating  is  simple  in  appearance 
but  is  difficult  to  make  for  it 
consists  of  a  great  many  paral- 
lel lines  (sometimes  as  many 
as  40,000  and  more  per  inch) 
°°  scratched  usually  upon  glass  or 
speculum  metal.  When  a  grat- 
ing is  placed  in  the  path  of  a 
beam  of  light  a  spectrum  re- 
sults, which,  unlike  a  prismatic 
spectrum,  has  a  constant  dis- 
persion and  is  therefore  called 


o, 


Fig.  9.-Young's  double-slit  ex-   *  n°rmal  Spectrum. 
periment  illustrating  the  princi-  To  explain  the  action  of  the 

diffraction  grating  we  shall  con- 

sider  only  two  of  the  very  large  number  of  slits  of  which 
the  grating  consists.     In  Fig.  9  has  been  sketched  an 


THE  PRODUCTION  OF  COLOR  27 

instantaneous  view  of  a  section  perpendicular  to  the 
two  slits.  A  source  of  light,  S,  has  sent  out  spherical 
monochromatic  waves,  successive  fronts  being  repre- 
sented by  the  full  lines.  The  dotted  lines  drawn  mid- 
way between  the  full  lines  must  then  represent  parts  of 
the  disturbance  exactly  in  opposite  phase  with  the  wave 
fronts.  If  the  diagram  be  considered  for  a  moment  as 
representing  a  surface  of  water  into  which  a  stone  has 
been  dropped  at  S,  then  the  full  lines  might  represent 
the  crests  of  the  waves  and  the  dotted  lines  the 
troughs.  One  wave  front  is  represented  as  contain- 
ing the  two  slits,  Si  and  S2.  It  was  stated  in  #  2  that 
according  to  Huyghen's  principle  any  point  on  a  wave  ( 
front  may  be  regarded  as  a  center  of  disturbance. 
The  wave  front  originally  proceeding  from  S  strikes 
the  screen  and  cannot  pass  through  excepting  for 
the  two  small  parts  of  this  wave  front  that  strikes 
the  openings  Si  and  S2.  These  set  up  the  two  sys- 
tems of  spherical  waves  represented  on  the  right 
hand  side  of  the  screen.  It  has  also  been  stated  (#  2) 
that,  whenever  two  or  more  systems  of  waves  travel 
in  the  same  medium  at  the  same  time,  interference 
results.  Wherever  two  wave  fronts  intersect  (two 
crests  in  the  water  analogy)  there  will  be  an  especially 
large  disturbance;  wherever  two  of  the  dotted  lines 
intersect  there  will  be  an  especially  large  disturbance 
(an  especially  deep  trough  in  the  water  analogy). 
Where  a  wave  front  meets  a  part  of  the  disturbance 
in  opposite  phase,  that  is  wherever  a  full  line  inter- 
sects a  dotted  line,  there  will  be  destructive  inter- 
ference and  consequently  no  motion.  The  heavy 
full  lines  have  been  drawn  through  those  points 
where  there  is  maximum  motion  and  the  heavy 
dotted  lines  through  those  points  where  there  is  no 
motion  and  therefore  no  light.  Now  suppose  these 


28  COLOR  AND  ITS  APPLICATIONS 

waves  be  permitted  to  impinge  upon  a  screen  repre- 
sented by  the  right  hand  edge  of  the  diagram.  There 
will  evidently  be  light  on  the  screen  wherever  a 
heavy  dotted  line  terminates.1  !ll^ 

Imagine  that  the  source  S  has  been  sending  out 
blue  light  and  is  now  exchanged  for  one  that  emits 
red  light.  A  very  similar  diagram  will  result  but  on 
account  of  the  longer  wave-length,  the  ends  of  the 
heavy  lines  will  no  longer  intersect  the  screen  at  the 
points  d  and  02;  there  will  still  be  light  at  00  as  is 
evident  from  the  symmetry  of  the  diagram,  but  the 
new  points  corresponding  to  Oi  and  02  will  lie  some- 
what farther  from  00  than  in  the  case  of  blue  light. 

Finally,  suppose  the  source  S  to  emit  white  light 
(light  of  all  visible  wave-lengths).  Then  there  will 
be  a  white  spot  at  00  and  spectra  at  d  and  02,  and 
the  blue  edge  of  these  spectra  will  be  nearest  00. 
In  other  words,  in  a  grating  spectrum  the  long  wave- 
lengths are  deviated  more  than  the  short  ones, 
which  is  opposite  to  the  condition  that  exists  in  prism 
spectra.  Further,  it  will  be  seen  that  the  distances 
in  the  spectra,  d,  02,  etc.,  are  proportional  to  the 
wave-length;  in  other  words,  that  a  grating  spectrum 
is  one  of  constant  dispersion.  (See  Plate  I.) 

The  spectra,  Oi,  are  said  to  be  of  the  first  order, 
02  of  the  second  order,  and  so  on.  It  is  also  evident 
that  if  more  heavy  lines  had  been  drawn  in  the  dia- 
gram the  positions  of  the  third  and  higher  order 
spectra  would  also  have  been  indicated.  It  will  be 
further  noted  that  the  higher  the  order  the  greater 
is  the  length  of  the  spectrum,  and  therefore  the 
greater  is  the  dispersion. 

The  colors  of  some  crystals  such  as  the  fiery  opal, 
and  of  insects  and  feathers  are  often  accounted  for 
by  the  phenomenon  of  diffraction.  On  looking  at 


THE  PRODUCTION   OF   COLOR  29 

an  arc  lamp  through  a  screen  door  or  the  meshes  of 
an  umbrella  top,  diffraction  spectra  are  seen.  Like- 
wise on  viewing  a  light  source  over  a  thin  edge  of 
an  opaque  object  held  close  to  the  eye,  a  colored 
fringe  is  seen.  These  bands  are  due  to  the  inter- 
ference of  light  waves.  Since  the  wave-length  of 
red  light  is  greater  than  that  of  violet  light,  the  red 
light  penetrates  further  into  the  shadow  than  the 
violet  light. 

The  first  gratings  were  made  by  Fraunhofer  in 
1821,  and  consisted  either  of  fine  wire  or  of  fine 
rulings  on  a  smoked  glass.  At  present  gratings  are 
ruled  by  means  of  a  diamond  on  glass  and  on  the 
bright  reflecting  surface  of  speculum  metal.  Row- 
land at  Johns  Hopkins  University  contributed  much 
toward  the  production  of  satisfactory  gratings.  On 
his  machine  as  high  as  110,000  lines  per  inch  can  be 
ruled,  but  usually  the  number  does  not  exceed  15,000 
or  20,000.  Cheap  copies  of  gratings  are  obtained  by 
flowing  a  film  of  celluloid  dissolved  in  amyl  acetate 
over  a  grating,  afterward  stripping  this  off  and  mount- 
ing it  between  plate  glasses.  It  is  evident  that  in 
the  case  of  rulings  upon  an  opaque  substance  such 
as  speculum  metal  the  spectra  must  be  produced  by 
reflection  and  not  by  transmission  as  discussed  above. 
The  individual  rulings  always  act  as  absorbing  bodies 
while  the  unruled  portions  either  reflect  or  transmit 
this  light. 

10.  Interference.  —  Other  color  phenomena  be- 
sides that  of  the  diffraction-grating  spectrum  arise 
from  the  process  of  interference.  Thin  films  of  trans- 
parent substances  such  as  oil  on  water,  soap  bubbles, 
thin  sheets  of  mica,  and  iridescent  crystals,  owe 
their  color  usually  to  the  interference  of  light  waves. 
If  a  slightly  convex  glass  surface  be  placed  upon  a 


30  COLOR  AND   ITS  APPLICATIONS 

plane  piece  of  glass,  colored  bands  known  as  Newton's 
rings  will  be  seen.  These  are  due  to  interference 
between  the  light  waves  reflected  from  the  upper 
and  lower  surfaces  respectively  of  the  thin  film  of 
air  of  varying  thickness  between  the  two  pieces  of 
glass.  When  white  light  is  incident  normally  at  the 
point  of  contact  the  colored  bands  are  circular  and 
concentric  with  the  point  of  contact.  These  bands 
are  violet  on  the  inside  and  red  on  the  outside  but 
at  a  short  distance  from  the  center  they  begin  to 
overlap  and  gradually  disappear.  If  monochromatic 
light  is  used  the  bands  appear  of  the  color  of  the  light 
and  many  bands  can  be  seen.  In  the  case  of  some 
crystals  such  as  chlorate  of  potash  and  fiery  opals, 
the  colors  are  found  to  be  very  pure.  The  colors  of 
insects  and  feathers  are  often  accounted  for  by  the 
phenomena  of  interference.  The  process  of  color 
photography  devised  by  Lippmann  (#  57)  is  based  upon 
the  principle  of  interference  of  light  waves. 

11.  Polarization. — Imagine  for  the  sake  of  sim- 
plicity a  beam  of  sunlight  emerging  from  an  aperture 
in  a  window  shade.  All  the  different  waves  that 
make  up  this  beam  are  traveling  in  the  same  gen- 
eral direction.  As  mentioned  previously,  light  waves 
are  transverse;  that  is,  the  particles  of  the  medium 
that  transmit  the  waves  move  to  and  fro  in  a  straight 
line  perpendicular  to  the  direction  in  which  the  beam 
is  traveling.  In  the  simplest  case  all  of  the  particles 
that  transmit  a  wave  move  so  as  always  to  be  in  one 
plane.  For  example  in  a,  Fig.  10,  the  particles  A,  B, 
Cj  Dy  etc.,  move  to  and  fro  along  perpendicular  paths 
but  the  wave  travels  horizontally.  If  such  a  wave 
could  be  seen  from  one  end  it  would  appear  simply  as 
a  short  straight  vertical  line  as  indicated  in  b.  Sup- 
pose that  the  beam  of  sunlight  emerging  through 


THE  PRODUCTION   OF  COLOR 


31 


the  window  shade  could  be  examined  minutely  end 
on.  We  would  then  see  something  similar  to  c  for  the 
different  waves  vibrate  in  all  possible  different  planes. 
Suppose  further  that  by  some  means  the  beam  could 
be  transformed  so  that  when  seen  end  on,  it  would 


a 


,B 


Fig.  10.  —  Diagrammatic  illustration  of  polarized  light. 

appear  like  d.  Such  a  beam  consisting  of  waves 
in  parallel  planes  is  said  to  be  a  plane-polarized 
beam,  and  curiously,  the  nomenclature  adopted  states 
that  such  a  beam  in  which  the  waves  all  move  in 
vertical  planes,  is  polarized  in  the  horizontal  plane. 
It  is  understood  that  these  graphical  diagrams  are 
used  for  the  sake  of  presenting  the  subject  pictorially 
and  with  no  claim  that  they  represent  actual  condi- 
tions. Nevertheless  if  the  actual  conditions  were 
such  the  results  of  experiments  would  readily  be 
explained  by  the  reasoning  used  here.  One  simple 
way  of  i polarizing'  a  beam  of  light  is  by  permitting 
it  to  fall  upon  a  plate  of  glass  so  that  the  angle  of 
incidence  is  57  degrees  as  shown  in  e.  The  unpolar- 
ized  beam  AB  will  be  separated  into  refracted  (BC) 


32  COLOR  AND   ITS  APPLICATIONS 

and  the  reflected  (BD)  beams,  and  the  former  will  be 
found  to  consist  mainly  of  waves  vibrating  in  the 
plane  of  the  paper,  as  indicated  by  the  cross  lines 
representing  the  path  of  the  particles,  while  the 
reflected  beam  will  be  found  to  consist  of  waves 
vibrating  perpendicular  to  the  plane  of  the  paper  as 
indicated  by  the  dots  representing  the  paths  seen 
end  on.  To  show  that  the  beam  BD  really  has 
different  properties  than  B  C  it  is  only  necessary  to 
attempt  to  reflect  each  from  another  piece  of  glass. 
If  the  second  piece  of  glass  be  placed  at  C  in  a 
position  similar  to  the  first,  the  beam  BC  will  pass 
through  but  if  it  be  rotated  about  C  through  an  angle 
of  90  degrees  keeping  its  angle  with  the  beam  con- 
stant it  will  be  found  that  the  beam  BC  will  not  be 
transmitted.  The  treatment  for  the  beam  BD  is 
obvious. 

Another  means  of  polarizing  a  beam  of  light  is 
by  permitting  it  to  pass  through  certain  crystals, 
such  as  tourmaline,  Iceland  spar  and  quartz.  If 
this  be  done  it  will  be  found  that  the  incident  beam 
is  divided  into  two  beams  polarized  at  right  angles 
to  each  other,  the  two  having  different  directions  in 
the  crystal,  different  velocities  and  different  prop- 
erties in  general.  One  of  these  beams  will  always 
be  found  to  obey  the  ordinary  laws  of  refraction,  for 
example,  that  the  incident  ray,  the  normal  to  the 
surface,  and  the  refracted  ray  all  lie  in  one  plane; 
the  other  beam  will  not  obey  the  foregoing  and  other 
simple  laws  generally.  The  former  is  therefore 
called  the  ordinary  ray  and  the  latter  the  extraor- 
dinary ray.  Nicol  devised  a  very  convenient  method 
of  separating  the  two  beams  when  produced  by  a 
crystal  of  Iceland  spar  as  illustrated  in  Fig.  11.  If 
a  rhomb  of  spar  be  cut  into  two  parts  along  the  plane 


THE  PRODUCTION   OF  COLOR 


33 


indicated  by  the  diagonal,  PPy  and  if  the  parts  be 
polished  and  cemented  together  with  Canada  balsam, 
the  paths  of  the  two  beams  will  be  as  indicated. 
The  Canada  balsam  has  a  refractive  in- 
dex intermediate  between  that  of  the  spar 
for  the  ordinary  and  extraordinary  rays. 
Therefore  the  ordinary  ray,  O,  being  inci- 
dent upon  the  balsam  layer  at  an  angle 
greater  than  the  critical  angle,  is  totally 
reflected  while  the  extraordinary  ray  is 
merely  slightly  refracted  by  the  layer  of 
balsam. 

That  the  beam  emerging  from  the 
Nicol  prism  is  really  plane-polarized  can 
be  shown  by  a  mirror,  as  before,  or  more   Fig.  11.— The 
simply,  by  a  second  Nicol  prism.     If  the      Nicol  prism 

for    obtaining 

second  prism  be  rotated  through  one  com-  piane-poiar- 
plete  revolution,  two  positions  will  be  lzed  llght* 
found  for  which  the  light  transmitted  by  both  prisms 
is  practically  of  the  same  intensity  as  that  transmitted 
by  one,  and  two  other  positions  will  be  found  for  which 
no  light  is  transmitted.  For  intermediate  positions  the 
intensity  of  the  light  will  be  less  than  that  transmit- 
ted by  one  Nicol.  When  the  maximum  amount  of 
light  is  transmitted  the  Nicols  are  said  to  be  *  parallel' 
and  when  no  light  is  transmitted  they  are  'crossed.' 

Some  substances,  such  as  quartz  (cut  in  a  certain 
manner),  sulphate  of  lime,  sugar  solution,  and  tur- 
pentine, behave  in  a  peculiar  manner  when  placed 
between  crossed  Nicols,  for  though  no  light  passes 
the  second  prism  before  the  introduction  of  the  other 
substance,  some  light  is  transmitted  when  the  sub- 
stance is  inserted  in  the  path.  If  monochromatic 
light  is  used  the  light  can  be  extinguished  by  rotating 
either  prism  to  the  right  or  left  by  an  amount  depend- 


34 


COLOR  AND  ITS  APPLICATIONS 


ing  upon  the  substance,  its  state  of  concentration  if 
in  solution,  and  the  thickness  of  the  layer  intro- 
duced. Such  layers  are  said  to  rotate  the  plane  of 
polarization.  If  various  monochromatic  lights  of  dif- 
ferent colors  are  used  in  succession  it  will  be  found 
that  the  prism  must  be  rotated  different  amounts  for 
the  different  colors  in  order  to  bring  about  a  total 
extinction  of  light  after  the  introduction  of  one  of 
the  substances.  Hence  if  such  a  substance  be  ex- 
amined in  white  light,  a  certain  position  of  the  prism 
will  extinguish  the  blue  light  but  permitting  the  re- 
maining colored  rays  to  pass  in  varying  proportions; 
at  another  position  yellow  will  be  extinguished  #nd 
the  remaining  rays  permitted  to  pass  in  varying  pro- 
portions, and  so  on.  The  transmitted  light  will  of 
course  have  a  different  color  in  each  case.  As 
quartz  is  one  of  the  chief  substances  having  this 
property,  the  following  table  is  given  to  show  the 
magnitude  of  the  rotation  in  degrees  produced  by 
two  thicknesses  of  quartz  for  light  of  different  wave- 
lengths. 


Thickness  of 
quartz 

Red 

Orange 

Yellow 

Green 

Blue 

Violet 

1  mm. 

18° 

22° 

24° 

30° 

32° 

42° 

7.5  mm. 

135 

161 

180 

218 

232 

315 

Another  instance  of  the  production  of  color  by 
polarization  is  that  of  the  examination  between 
crossed  Nicols  of  certain  other  crystals  that  trans- 
form plane-polarized  light  into  so-called  circularly  or 
elliptically-polarized  light.  Many  minerals  occuring  in 
Nature  have  either  this  property  or  that  of  rotating 
the  plane  of  polarization,  and  an  entire  system  has 
been  developed  for  determining  what  substances  are 


THE  PRODUCTION   OF  COLOR  35 

present  in  a  given  rock  by  noting  the  color  effects 
produced  by  a  thin  section  of  the  rock  in  a  micro- 
scope equipped  with  two  Nicol  prisms. 

12.  Reflection,  Absorption,  Transmission.  —  Ordi- 
nary colors,  which  are  encountered,  are  produced  by 
selective  reflection  or  transmission.  Since  most  sub- 
stances do  not  have  the  property  of  reflecting  the 
same  proportions  of  all  light  rays  received  they  are 
said  to  be  jselective  in  their  reflection.  A  red  fabric 
has  the  ability  to  reflect  chiefly  the  red  rays  of  the 
visible  spectrum;  therefore  when  white  light  falls 
upon  it  only  the  red  rays  are  reflected  while  the 
remaining  visible  rays  are  absorbed.  By  this  process 
of  selective  reflection  the  colors  of  pigments  and  most 
of  the  colors  in  Nature  are  produced.  The  same 
remarks  apply  to  the  production  of  color  by  selective 
transmission.  Colors  produced  by  these  means  are 
not  as  pure  as  the  colors  of  the  spectrum.  Each 
of  the  latter  consists  practically  of  a  single  wave- 
length and  are  said  to  be  monochromatic,  while  the 
colors  ordinarily  encountered  in  Nature  are  im- 
pure, consisting  of  rays  of  a  considerable  range  of 
wave-lengths. 

In  Fig.  12  are  shown  diagrammatically  the  spectral 
analyses  of  five  common  pigments.  For  each  pig- 
ment the  reflecting  power  (i.e.  the  per  cent  of 
energy  reflected)  was  determined  for  all  wave-lengths 
in  the  visible  spectrum.  The  results  plotted  against 
the  corresponding  wave-lengths  are  represented  by 
the  full  lines  in  the  diagram.  The  dotted  curves 
represent  roughly  the  relative  light  values  and  are 
obtained  from  the  full  curves  by  multiplying  the 
energy  values  represented  by  the  ordinates  by  their 
relative  abilities  to  produce  light  sensation  (#4). 
A  similar  discussion  applies  to  the  same  colors  pro- 


36 


COLOR  AND   ITS  APPLICATIONS 


duced  by  transmission.  These  analyses  will  be  dis- 
cussed further  in  Chapter  V.  (See  Figs.  122  and  123.) 
The  character  of  the  surface  of  a  pigment  and  the 
density  of  the  coloring  matter  influence  the  appear- 
ance of  a  color.  If,  for  instance,  a  red  aniline  dye 
solution  be  deposited  upon  a  fabric  by  means  of  an 
air  brush,  the  pigment  under  some  conditions  will 


PURPLE 


BLUE 


GREEH 


YELLOW 


RED 


0.40  0.50  0.60  0.70 

_Jl,   WAVELE.MGTH 

Fig.  12.  —  Analyses  of  ordinary  colors. 


be  deposited  in  the  form  of  a  fluffy  powder  whose 
appearance  is  a  deep  velvety  red.  The  purity  of  the 
red  is  largely  due  to  multiple  reflections,  for  the  light 
is  able  to  penetrate  an  appreciable  distance  into  the 
medium  (#64,  75),  and  at  each  reflection  it  becomes 
purer — that  is,  more  nearly  monochromatic.  If  the 
solution  were  applied  by  means  of  an  ordinary  brush, 
the  deposit  would  not  have  been  so  porous  and  the 
appearance  of  the  color  would  not  have  been  such 
a  deep  red.  Another  instance  of  the  effect  of  mul- 
tiple reflections  on  the  color  of  light  is  found  in  a 
gold-lined  goblet.  All  are  familiar  with  the  color 
of  gold  plating.  This,  however,  becomes  much  more 


THE  PRODUCTION   OF  COLOR  37 

reddish  when  inside  a  goblet,  owing  to  the  purifying 
effect  of  multiple  reflections. 

The  color  of  transparent  media  depends  upon  the 
depth  of  the  coloring  matter.  For  instance,  a  hollow 
glass  wedge,  when  filled  with  an  aqueous  solution  of 
ethyl  or  methyl  violet,  will  appear  bluish  at  the  thin 
end  and  reddish  at  the  thick  end.  Cyanine  is  an- 
other dye  that  exhibits  dichroism,  which  is  the  name 
applied  to  the  foregoing.  The  different  appearances 
of  silk  and  woolen  fabric  dyed  in  the  same  solution 
are  due  largely  to  the  difference  in  the  character  of 
the  surfaces.  Many  of  these  instances  could  be 
cited  here,  but  the  details  will  be  treated  in  succeed- 
ing chapters. 

13.  Color  Due  to  Scattered  Light. --That  light 
is  changed  in  color  by  being  scattered  by  fine  par- 
ticles is  a  fact  observed  daily.  Tyndall,  by  precipi- 
tating clouds  of  vapor,  observed  that  as  the  particles 
increased  in  size  the  bluish  color  of  the  clouds  dis- 
appeared. Smoke  from  the  end  of  a  cigar  appears 
bluish,  yet  the  smoke  exhaled  appears  whitish;  this 
latter  is  perhaps  caused  by  an  increase  in  the  size 
of  the  particles,  due  to  the  condensation  of  moisture. 
Rayleigh  1  has  mathematically  treated  the  problem  of 
scattered  light  and  experimented  with  a  sulphur 
precipitate.  He  noted  a  polarizing  effect  which  is  of 
interest.  J.  J.  Thomson  2  treated  the  scattering  due 
to  small  metallic  spheres  theoretically.  Garnett 3  con- 
cluded that  colored  glasses  owe  their  colors  to  the 
presence  of  microscopic  spheres  of  the  metal  of  the 
coloring  agent.  Colloidal  solutions  appear  to  act  in 
the  same  manner.  Mie  4  has  extended  Garnett's 
theory  very  considerably;  however,  the  exact  explana- 
tion of  the  color  of  glasses  is  still  somewhat  disputed. 

The  color  of  daylight  is  of  special  interest  because 


38 


COLOR  AND   ITS  APPLICATIONS 


it  is  our  most  common  illuminant.  Light  from  the 
sky  consists  chiefly  of  scattered  sunlight  in  daytime. 
If  it  were  not  for  the  finely  divided  particles  of  matter 
in  our  atmosphere,  the  sky  would  be  quite  as  dark 
by  day  as  by  night.  When  the  particles  are  of  a 
size  comparable  with  the  wave-length  of  light  rays, 
they  produce  considerable  scattering  of  the  latter. 
This  scattering  of  light  is  selective,  the  rays  of  short 
100 


.  WAVE  LENGTH 


Fig.  13.  —  Showing  the  variation  in  the  spectral  character  of  sunlight  due  to 
atmospheric  absorption. 

wave-length  being  scattered  in  greater  amounts  than 
those  of  longer  wave-length;  this  accounts  for  the 
bluish  color  of  the  sky  (Fig.  5).  Sunlight  is  termed 
white  light,  but  in  many  cases  throughout  this  book 
the  term  '  white'  light  is  used  to  indicate  the  total 
light  from  an  illuminant  emitting  all  visible  rays. 
Where  necessary,  the  two  meanings  are  differentiated. 
Direct  sunlight,  however,  undergoes  a  change  in 
color  as  the  altitude  of  the  sun  changes,  on  account 
of  the  greater  thickness  of  air,  more  or  less  laden 
with  smoke,  dust,  vapor,  and  ice,  through  which 
the  light  must  pass  at  the  lower  altitudes.  This 
change  in  color  caused  by  the  combined  actions  of 


THE  PRODUCTION   OF   COLOR 


39 


absorption  and  scattering,  is  such  as  to  depress  the 
blue,  and  hence  the  sun  appears  redder  as  it  ap- 
proaches the  horizon.  All  the  brilliant  colors  of 
sunset  are  due  to  the  foregoing  phenomena.  The 
effect  of  different  masses  of  air  upon  the  relative 
amounts  of  energy  in  each  wave-length  which  reach 
a  given  point  on  the  earth's  surface  has  been  deter- 
mined by  Abney;  his  data  are  reproduced  in  Fig.  13. 
Of  course  the  results  obtained  will  depend  upon  the 
purity  of  the  atmosphere.  Over  a  smoky  industrial 
city,  the  sun,  when  near  the  horizon,  almost  daily 
appears  a  fiery  red.  Often  the  absorption  is  so  great 
that  the  sun  disappears  from  view  long  before  it 
actually  sinks  below  the  horizon. 

14.  Color  Sensations  Produced  by  Colorless 
Stimuli.  —  Colors  are  often  visible  to  a  careful 
observer  when  not  produced  by  any  of  the  methods 
already  mentioned.  If  a  disk  composed  of  black  and 
white  be  rotated  at  the  proper  rate  —  moderately 
slow — colors  appear  upon  the  leading  and  lagging 
edges  of  the  sectors. 

In  other  words  when 
the  black  and  white  stimuli 
precede  or  follow  each  other 
at  certain  intervals,  colors 
are  produced  instead  of 
gray.  Fechner,  in  1838,  was 
perhaps  the  first  to  describe 
these  subjective  colors,  and 
his  name  was  later  applied 
to  the  phenomenon  by 

Briicke.      Many   have    Stud-     Fig.  14.—  Benham  disk  for  producing 

ied  the  problem  and  there      £**£  *£** means  of  bteck 

is  a  general  agreement  as  to 

the  results  obtained,  though  there  is  no  such  agreement 


40  COLOR  AND   ITS  APPLICATIONS 

as  to  their  explanation.  Benham,5  in  1894,  produced 
a  disk  different  from  those  used  by  preceding  investi- 
gators and  made  an  attempt  to  solve  the  problem. 
One  form  of  his  disk,  illustrated  in  Fig.  14,  shows 
the  colors  in  a  striking  manner  when  rotated.  The 
phenomena  can  only  be  briefly  discussed  here.  In 
general,  when  black  is  followed  by  white  at  a  mod- 
erate speed,  a  sensation  of  red  results,  but  if  white 
be  followed  by  black,  a  sensation  of  blue  is  experi- 
enced. By  introducing  various  angular  intervals, 
as  is  done  in  the  Benham  disk,  sensations  of  inter- 
mediate colors  are  aroused.  On  rotating  the  disk 
in  one  direction  the  blue  sensation  is  aroused  in  the 
inner  ring  and  red  in  the  outer;  on  reversing  the 
disk,  the  colors  aroused  are  also  reversed  in  their 
order.  The  phenomena  are  interesting  and  have 
received  a  great  deal  of  attention,  though,  as  already 
stated,  there  is  no  general  agreement  as  to  their 
complete  explanation.  No  doubt  retinal  inertia  and 
the  difference  in  the  rates  of  growth  and  decay  of 
the  color  sensations  are  important  factors  in  the 
production  of  these  so-called  subjective  colors.  Often, 
when  suddenly  moving  the  eye  over  black  and  white 
surfaces  in  the  field  of  vision,  these  colored  effects 
are  perceptible.  A  simple  disk  for  showing  the 
Fechner  colors,  though  not  as  effective  as  the  Benham 
disk,  is  one  containing  plain  black  and  white  sectors. 
On  rotating  such  a  disk  at  a  certain  speed  it  will 
appear  of  a  greenish  hue,  but  at  a  somewhat  more 
rapid  rate  of  rotation  it  appears  a  reddish  hue.  Helm- 
holtz  used  a  white  disk  upon  which  was  painted  a 
black  spiral.  Rood  6  used  an  opaque  disk  with  four 
open  sectors,  each  of  seven  degrees.  Through  this 
rotating  disk  he  viewed  a  clouded  sky.  With  a  rate 
of  nine  revolutions  per  second  the  sky  appeared  a 


THE  PRODUCTION   OF   COLOR  41 

deep  crimson  hue,  except  for  a  small  spot  in  the 
center  of  the  visual  field,  which  remained  constantly 
yellow.  This  latter  is  probably  due  to  retinal  dif- 
ferences. The  center  of  the  retina,  called  the  *  yellow 
spot,'  is  known  to  exhibit  selective  absorption.  At 
eleven  and  one-half  revolutions  per  second  the  field 
appeared  bluish-green. 

15.  Fluorescence  and  Phosphorescence.  —  Usually 
the  radiant  energy  absorbed  by  bodies  is  trans- 
formed into  heat  energy.  There  are  many  sub- 
stances, however, — some  solid,  some  liquid,  and  some 
gaseous, — that  have  the  property  of  absorbing  radiant 
energy  of  certain  wave-lengths  and  of  emitting  it 
again  after  transforming  it  into  radiant  energy  of 
other  wave-lengths  (nearly  always  longer  than  those 
absorbed). 

The  name  fluorescence  was  derived  from  fluor  spar 
(calcium  fluoride),  which  has  long  been  known  to 
possess  the  property  of  emitting  light  rays  of  a  dif- 
ferent color  than  that  of  the  rays  with  which  it  is 
illuminated.  Strictly  speaking,  the  term  applies  to 
the  phenomenon  only  when  it  ceases  immediately 
after  the  exciting  light  is  extinguished;  in  this  sense 
it  is  applicable  to  liquids  and  gases  only.  In  solids 
the  phenomenon  usually  continues  for  some  time 
after  the  exciting  light  has  been  shut  off.  This 
prolonged  emission,  which  in  some  cases  lasts  for 
hours,  is  termed  phosphorescence.  The  phenomena 
of  fluorescence  and  phosphorescence  are  sometimes 
classified  under  the  more  general  term  luminescence. 
Though  there  are  comparatively  few  substances  that 
exhibit  the  phenomenon  to  a  marked  degree,  it  is 
difficult  to  find  materials  that  do  not  show  the  prop- 
erty slightly.  This  is  readily  seen  by  examining 
ordinary  substances  in  an  intense  spectrum  of  the 


42  COLOR  AND  ITS  APPLICATIONS 

light  from  a  quartz  mercury  arc  produced  by  means 
of  a  quartz  optical  system.  In  examining  the  fluor- 
escent light  it  is  often  convenient  to  look  through  a 
glass  of  such  a  color  that  it  will  transmit  the  fluor- 
escent light  rays  and  absorb  the  exciting  rays.  The 
phenomenon  is  influenced  by  temperature,  and  in 
most  cases  the  phosphorescence  is  temporarily  in- 
creased in  brightness  by  the  application  of  heat  or 
red  and  infrared  rays,  but  the  duration  of  this  in- 
creased brightness  is  usually  brief,  as  the  phos- 
phorescence is  rapidly  extinguished  by  these  agencies. 
The  phenomenon  is  of  great  interest  to  the  scientist 
and  also  in  smaller  degree  to  the  colorist  and  light- 
producer.  Fluorescent  phenomena  play  a  part  in 
the  appearance  of  certain  colors  (#  75),  and  there  are 
possibilities  for  utilizing  the  phenomenon  for  the 
production  of  light  for  practical  purposes. 

Some  examples  of  fluorescence  and  phosphores- 
cence should  be  of  interest.  Sunlight  and  the  light 
from  carbon,  mercury,  zinc,  iron,  and  silicon  arcs 
are  rich  in  ultra-violet  rays  which  ordinarily  are  the 
most  active  in  exciting  fluorescence  and  phosphores- 
cence. In  these  cases  it  is  convenient  to  remove 
most  of  the  yellow,  orange,  red  and  infrared  rays  from 
the  exciting  beam  by  means  of  a  dense  violet  glass 
and  a  water  cell.  Uviol  blue  glass  is  especially  satis- 
factory. Water  of  a  few  centimeters  depth  is  practi- 
cally opaque  to  infrared  rays,  but  when  pure  is  quite 
transparent  to  ultra-violet  rays.  The  fluorescence, 
consisting  in  general  of  rays  of  longer  wave-length 
than  the  exciting  light,  can  be  viewed  through  a  glass 
of  proper  color  without  being  confused  by  the  color 
of  the  exciting  light.  Fluorescent  materials  are  valu- 
able in  visually  investigating  the  ultra-violet  region 
of  a  spectrum;  for  example,  uranium  glass  is  very 


THE  PRODUCTION   OF   COLOR  43 

convenient  for  focussing  a  spectrograph  for  the  in- 
visible ultra-violet  rays. 

Aesculine,  which  is  ordinarily  transparent  in  solu- 
tion or  in  a  gelatine  film,  fluoresces  a  bluish  color 
under  strong  ultra-violet  excitation.  It  is  valuable  in 
photography  as  a  screen  for  absorbing  ultra-violet  rays. 
A  solution  of  fluorescein  or  uranin  is  useful  in  demon- 
strating the  path  of  light  through  various  optical 
systems.  It  is  best  to  prepare  first  a  solution  of 
moderate  concentration  and  add  this  drop  by  drop 
to  the  tank  of  clear  water.  Of  course  the  difference 
between  the  refractive  index  of  the  water  and  that 
of  air  must  be  taken  into  consideration  when  demon- 
strating the  path  of  light  through  a  given  optical 
system  immersed  in  water.  Kerosene,  an  alcoholic 
solution  of  chlorophyl,  anthracene,  and  many  of  the 
organic  dyes  exhibit  the  phenomenon  of  fluorescence. 
In  cases  of  strong  excitation  the  emission  of  light 
rays  by  some  of  these  substances  continues  for  some 
time  after  the  exciting  light  is  cut  off. 

Substances  that  exhibit  prolonged  phosphorescence 
are  chiefly  the  alkaline  earth  sulphides.  Balmain's 
paint,  an  impure  sulphide  of  calcium,  is  one  of  the 
most  active  and  least  expensive.  Its  phosphorescent 
light  is  of  a  bluish  color.  Other  phosphorescent 
sulphides  emit  light  of  various  colors,  and  by  com- 
bining various  ones,  nearly  any  desirable  color  can 
be  obtained.  For  demonstration  purposes  beautiful 
designs  can  be  made  by  the  use  of  various  phos- 
phorescent media  which  emit  light  of  different  colors. 
The  chief  difficulties  which  limit  the  use  of  phos- 
phorescent substances  are  the  scarcity  of  the  exciting 
rays  in  ordinary  light  sources  and  the  rapid  decay 
of  the  intensity  of  the  emitted  light  after  the  excitation 
has  been  cut  off. 


COLOR  AND   ITS  APPLICATIONS 


One  instance  of  a  commercial  application  of  fluor- 
escence as  a  light  source  is  the  adaptation  of  a  rhoda- 
mine  reflector  to  the  mercury  vapor  arc  lamps  by 
Peter  Cooper  Hewitt.  The  reflector  consists  of  a 
white  paper  base  upon  which  the  rhodamine  layer  is 
placed.  The  latter  is  apparently  protected  by  a  trans- 
parent varnish.  On  focussing  the  spectrum  from 
the  quartz  mercury  arc  upon  it  and  viewing  it  through 
a  red  glass,  it  is  seen  that  practically  all  rays  from 
green  to  the  extreme  ultra-violet  excite  the  red  fluor- 


I  I 

la      ^ 


040 


0.45 


0.50 


Q.55 
WAVE  LENGTH 


0.65 


070 


Fig.  15.  —  Diagrammatic  illustration  of  the  action  of  the  rhodamine  fluorescent. 

reflector. 

escence.  That  is,  a  photograph  through  a  red  filter 
of  this  projected  spectrum  appears  quite  the  same  as 
an  actual  spectrogram  of  the  light  from  the  quartz 
mercury  arc.  The  action  of  the  reflector  is  dia- 
grammatically  shown  in  Fig.  15.  The  heavy  vertical 
lines  represent  the  visible  lines  of  the  mercury  spec- 
trum, their  lengths  being  approximately  proportional 
to  their  energy  intensities;  curve  R  represents  the 
reflection  curve  of  the  rhodamine  dye,  and  F  the 
fluorescent  light.  It  is  seen  that  a  gap  in  the  spec- 
trum of  the  light  from  a  mercury  arc  equipped  with 
this  reflector  exists  in  the  blue-green  region.  While 
this  reflector  greatly  improves  the  appearance  of 
colored  objects  illuminated  by  the  mercury  arc,  the 
light  is  still  unsatisfactory  for  accurate  color  work, 
owing  to  the  gap  mentioned  and  to  the  strong  emission 


THE   PRODUCTION   OF   COLOR  45 

lines.  In  Fig.  16  is  given  the  spectrophotographic 
analysis,  by  means  of  a  prism  instrument,  of  the  prop- 
erties of  the  rhodamine  reflector.  The  slit  of  the 
spectrograph  was  purposely  ad- 
justed  somewhat  wider  than 
usual,  on  account  of  the  long 
exposure  (several  hours)  required 

to  obtain  a  spectrogram  of  the  d 

fluorescent  light,  a  represents 
the  reflection  of  the  reflector 
for  tungsten  light;  &,  the  spec- 
trum  of  the  tungsten  light; 
c,  the  mercury  spectrum;  d,  the  ultraviolet 
reflection  of  the  rhodamine  re- 

fleeter  illuminated  by  the  total  rhodamine  fluorescent  re- 
light from  the  mercury  arc ;  e,  the 

mercury  spectrum  (shorter  exposure);  /,  the  isolated 
mercury  green  line  produced  by  a  special  filter  de- 
scribed later;  g,  the  fluorescence  spectrum  excited  by 
the  green  line,  the  latter  also  appearing  owing  to  diffuse 
reflection  from  the  fluorescent  reflector.  This  re- 
flector furnishes  red  rays  to  the  mercury  vapor  lamp, 
when  so  equipped,  at  the  expense  of  practically  all  the 
other  rays.  The  scheme  is  an  ingenious  one,  and 
probably  paves  the  way  for  other  practical  uses  of 
the  phenomenon  of  fluorescence. 

In  the  study  and  practical  use  of  fluorescent 
substances  the  solvent  is  of  importance,  owing  to  the 
influence  upon  the  intensity  of  the  fluorescence. 
Knoblauch 7  investigated  the  subject,  obtaining  the 
results  given  in  Table  III.  The  figures  ranging  from 
one  to  eleven  indicate  the  order  of  the  intensity, 
eleven  being  the  most  intense.  -The  table  is  also 
of  interest  in  suggesting  a  variety  of  solvents  for  dyes 
when  these  may  be  unknown  to  the  experimenter. 


COLOR  AND   ITS  APPLICATIONS 


TABLE  HI 
Effect  of  Solvent  upon  the  Intensity  of  Fluorescence 


• 

I 

« 

.« 

o 

I 

*c3 

1 

« 

% 

8 

| 

w 

Acetone 

| 

•a 

| 

,0 

1 

! 

13 

t 

<J 

IH 

O 

1 

! 

Gelatine 

-3 
& 

•3 

1 

<o 
PQ 

Aesculine 

3 

H 

3 

3 

3 

1 

3 

9! 

Anthracene  

4 

3 

ft 

4 

4 

5 

ft 

ft 

B.  PhenylnapMhyl^rnin 

5 

5 

3 

5 

4 

91 

1 

1 

1 

Chrysaniline 

1 

?! 

3 

Chrysolin      

9! 

3 

3 

3 

3 

1 

Eosine  (sodium) 

1 

9, 

6 

5 

4 

3 

Fluorescein  (lithium)  

a 

3 

5 

4 

1 

Fluorescene     .    ... 

1 

91 

3 

Petroleum 

5 

4 

3 

B 

6 

6 

Phenosafranine  
Magdala  red      .              .        ... 

i 

6 

7 
4 

9 
4 

11 

9 

10 
3 

4 

3 

1 

2 
1 

Cur  cumin 

1 

91 

3 

4 

Phenanthrene  

1 

9! 

16.  Useful  Filters.  -  -  Very  often  in  the  study  of 
color  phenomena  monochromatic  light  is  desired,  or 
ultra-violet  and  infrared  regions  of  the  spectrum  must 
be  isolated.  For  these  reasons  various  filters  which 
have  been  found  convenient  will  be  described. 

For  obtaining  intense  monochromatic  light  the 
quartz  mercury  arc  is  a  valuable  source.  Filters 
can  be  prepared  for  isolating  the  various  lines.  The 
filters  can  be  made  of  dyed  gelatine  and  cemented 
between  glass  plates  (or  quartz,  if  necessary)  with 
Canada  balsam,  or  the  dyes  can  be  dissolved  in  a 
proper  solvent  and  used  in  glass  or  quartz  cells.  The 
filters  can  be  tested  visually  by  means  of  a  spectro- 
scope or  photographically  with  a  panchromatic  plate 
and  a  spectrograph. 


THE   PRODUCTION    OF   COLOR  47 

For  isolating  the  mercury  yellow  lines,  0.5790  and 
0.5764/1,  chrysoidine  and  eosine  are  satisfactory. 

For  isolating  the  green  mercury  line,  0.5461, 
neodymium  ammonium  nitrate  and  either  potassium 
bichromate  or  eosine  form  an  excellent  combination. 
The  former  absorbs  the  yellow  lines,  and  the  latter  the 
blue  lines.  Neptune  green  S  and  chrysoidine  have 
been  recommended  for  the  purpose,  but  the  author 
has  found  the  former  method  more  satisfactory.  A 
cell  of  water  will  cut  off  the  infrared  rays  satisfactorily 
for  most  cases  when  this  is  necessary. 

For  isolating  the  blue  line,  0.4359,  cobalt  blue 
glass  and  aesculine  or  sulphate  of  quinine  form 
a  satisfactory  combination.  The  lines  0.4047  and 
0.4078 JJL  can  be  practically  isolated  by  a  combination 
of  methyl  violet  and  sulphate  of  quinine  in  separate 
solutions.  The  line  0.3984  is  transmitted  to  a  slight 
extent. 

The  ultra-violet  line,  0.3650,  can  be  practically 
isolated  by  methyl  violet  4/£  and  nitrosodimenthyl 
aniline,  methyl  violet  and  acid  green,  or  resorcine 
blue  and  aniline  green. 

By  the  use  of  other  line  spectra  and  ruby  glass 
or  red  dyes,  monochromatic  red  light  can  be  readily 
obtained. 

R.  W.  Wood  has  used  a  combination  of  strong 
cobalt-blue  glass  and  a  strong  yellow,  such  as  a  satu- 
rated solution  of  bichromate  of  potash,  for  isolating 
the  infrared  rays  beyond  0.69 M.  In  photographs  of 
a  landscape  through  this  filter  the  sky  appears  com- 
paratively black  and  the  foliage  white. 

Only  the  near  infrared  rays  and  no  visible  rays 
are  transmitted  by  a  solution  of  iodine  in  carbon 
bisulphide  in  a  cell  whose  sides  are  composed  of 
dense  red  glass.  Wood  has  used  lenses  coated  with 


48 


COLOR  AND   ITS  APPLICATIONS 


a  thin  film  of  chemically  deposited  silver  for  ultra- 
violet photography.  Such  a  film  is  opaque  to  all  rays 
excepting  a  narrow  region  in  the  vicinity  of  0.32 /z. 
The  formulae  for  silvering  are  readily  found  in  any 
recipe  book. 

Many  of  the  organic  dyes  and  colored  glasses  are 
useful  as  filters,  depending  upon  the  requirements. 
It  is  possible  by  a  careful  choice  of  filters  and  light 
sources  to  isolate  any  region  of  the  spectrum  desired. 
Photographic  plates,  owing  to  their  diversity  in  spectral 
sensitivities,  are  valuable  assets  in  some  problems. 
Filters  are  often  much  more  satisfactory  in  providing 
monochromatic  illumination  than  the  spectroscope, 
owing  to  the  much  greater  intensities  of  radiation 
obtainable. 

An  experiment  which  is  very  useful  and  educa- 
tive is  the  comparison  of  two  yellows  of  different 
spectral  compositions.  A  solution  of  potassium  bi- 
chromate in  water  transmits  yellow  rays.  If  to  a  por- 
tion of  this  solution  an  aqueous 
solution  of  neodymium  ammo- 
nium nitrate  be  added,  the  spec- 
tral yellow  rays  are  no  longer 
transmitted.  The  two  solutions 
(c  and  dj  Fig.  17)  appear  yellow 
in  daylight.  If  not  exactly  of  the 
same  color,  they  can  be  readily 
brought  to  the  same  appearance 
by  the  use  of  more  or  less  of 
the  potassium  bichromate  in 
one  of  the  solutions  or  by  the 
addition  of  one  of  the  yellow  or 
orange  dyes.  It  has  been  said  that  color  depends 
upon  the  wave-length  of  light.  However,  color  can 
not  always  be  taken  as  an  indication  of  wave-length, 


v 


Fig.  17. —  Screens  for  pro- 
ducing lights  of  the  same 
hue  but  differing  in  spec- 
tral character. 


THE   PRODUCTION   OF   COLOR  49 

because,  as  in  this  case,  the  two  solutions  appear  of 
the  same  color  —  yellow;  yet  when  examined  by 
means  of  the  spectroscope  one  (d)  is  found  to  trans- 
mit green,  yellow,  and  red  rays,  while  the  other  (c) 
transmits  no  yellow  rays — only  the  green  and  the  red 
rays.  The  latter  is  said  to  produce  a  subjective  yel- 
low. The  transmissions  of  the  two  solutions  are  shown 
in  Fig.  17  compared  with  the  spectrum  of  the  mer- 
cury arc  (g).  It  is  seen  that  the  absorption  band  of 
solution  c  falls  in  the  same  region  as  the  yellow  mer- 
cury lines,  0.5790  M  and  0.5764  M,  so  that  these  yellow 
lines  will  not  be  transmitted  by  it.  Therefore,  since 
the  yellow  solutions  are  not  transparent  to  the  rays  of 
shorter  wave-length,  the  solution  containing  the  neo- 
dymium  ammonium  nitrate  (c)  will,  when  illumi- 
nated by  the  mercury  arc,  only  transmit  the  green  line, 
0.5461,  as  shown  in  /.  On  viewing  a  mercury  arc 
through  each  of  the  two  solutions  this  is  readily 
verified;  one  solution  then  appears  a  brilliant  green, 
while  the  other  remains  yellow  in  color.  The  color 
of  the  green  mercury  line  can  readily  be  matched 
by  a  combination  of  colored  glasses  and  dyes.  The 
transmission  of  this  combination  filter  is  shown  in 
e.  The  two  yellow  solutions  c  and  d  were  made  to 
match  the  yellow  sodium  lines,  0.5890  and  0.5896, 
which  are  shown  (unresolved)  in  b.  These  lights,  of 
the  same  color  but  of  different  spectral  character, 
obtained  with  these  solutions  and  properly  chosen 
illuminants,  were  used  in  various  interesting  experi- 
ments to  be  discussed  later  (#37). 

An  exhaustive  spectrophotographic  treatment  of 
the  transmission  spectra  of  filters  is  outside  the 
scope  of  this  treatise,  but  a  number  of  spectrograms 
of  useful  filters  and  ordinary  glasses  are  given  in 
Figs.  18  and  19.  Uhler  and  Wood8  have  prepared  a 


U/trav/o/et 


Ultraviolet 


Visible 

a.  Tungsten  Incandescent  Lamp 

b.  CarbonArc 

c.  Iron  Arc 

cj.  Quartz  Mercury  Arc 

e.  Green  6/ass  (dense) 

f.  Signal  Blue  (medium) 

g.  "      Green(medium) 

h.   Canary  Yellow  (light) 
L  Orange  Gelatine  (light) 
j.  Green          »        (light) 
k.  Blue  "      (medium^ 

I.  Purple         "     (medium) 
m.  Cobalt    Blue  Class 
n.  Clear  Glass 
o.  Iron  Arc  (bare) 

p.  Signal  Green  (medium) 

q.  Celluloid 

r.  Cobalt  B/ue  Class 

s.  Clear  Glass 

t.  Canary  Yellow  (light) 
u.  Quartz  Mercury  Arc 
v.  Iron  Arc 

V/sib/e 


Fig.  18.  —  Ultra-violet  spectra. 
50 


U/travioIet  \  v/s/b/e 


mill  mi  mi) 


muni  mm 


nimnnm 


I    I     I 


a.  Quartz  Hercury  Arc 

b.  Aescul/ne 

c.  Acid  Green  +  Ethyl  VioJet 
d  . 

e.  Methyl  Violet+Hitrosodimenthyl  aniline 

f.  Acid  Green 

g.  Aniline  Green 
h.  Distilled  Water 
i.  A I  rubidium 

j.  Xy/er?e  Red 
k.  Rosazeme 
I.  Aniline  Red 
m.Acid  Green 

/?.  Aniline  Green 

a.  Quartz  Mercury  Arc 

p.  Anil  me  Yellow 

q.  Fluorescein 

r.  Tartrazme 

^s.  Uranin 

t.  Orange  G 

u.  Aurantia 

v.  Quartz  Mercury  A  rc- 


Ultraviolet  k  V/sible 


Fig.  19. — Ultra-violet  spectra. 


52  COLOR  AND   ITS  APPLICATIONS 

valuable  atlas  of  absorption  spectra,  and  C.  E.  K. 
Mees  9  has  prepared  a  similar  treatise  dealing  with 
organic  dyes.  The  spectrograms  in  Fig.  18  and  those 
from  i  to  v  in  Fig.  19  were  photographed  on  Cramer 
spectrum  plates,  which  are  sensitive,  but  in  varying 
degrees,  to  all  visible  and  ultra-violet  rays  transmitted 
by  quartz.  Spectrograms  a  to  h  (Fig.  19)  inclusive 
were  made  on  an  ordinary  plate  not  appreciably  sen- 
sitive, to  rays. of  longer  wave-length  than  0.48 /x. 

In  Fig.  18,  &,  c,  dy  show  the  spectra  of  sources  rich 
in  ultra-violet  rays.  The  next  group,  e  to  o  inclusive, 
shows  the  transmission  of  common  glasses  for  the 
radiation  from  an  iron  arc.  The  last  group  shows  the 
transmission  of  some  of  the  same  glasses  for  the 
radiation  from  the  quartz  mercury  arc. 

In  Fig.  19  are  shown  the  transmissions  of  various 
special  screens.  These  screens  are  all  cemented 
between  two  polished  glass  plates,  each  one-eighth 
inch  in  thickness.  The  glass  is  transparent  to  rays 
as  short  as  0.350/z,  but  begins  to  absorb  at  this  point, 
becoming  practically  opaque  to  ultra-violet  rays  shorter 
than  0.300 /x.  Some  of  the  first  spectrograms  illustrate 
the  ability  of  specially  prepared  filters  to  isolate  a 
narrow  region  of  the  ultra-violet  spectrum  at  0.365/1- 
For  instance,  acid-green  transmits  this  ultra-violet 
line,  but  also  transmits  much  visible  light  not  shown 
in  the  spectrogram  on  account  of  the  scarcity  of 
the  transmitted  visible  rays  in  the  spectrum  of  the 
mercury  arc.  However,  by  combining  with  this  screen 
a  visual  complimentary  dye,  such  as  ethyl  violet  (a 
purple),  which  likewise  transmits  line  0.365/x  but 
practically  none  of  the  visible  rays  transmitted  by 
the  acid-green,  a  visually  opaque  screen  is  produced 
which  transmits  rays  near  0.365  /x  quite  readily.  In 
much  the  same  manner  combination  screens  are 


THE   PRODUCTION    OF   COLOR  53 

devised  for  isolating  any  region  of  the  spectrum. 
There  is  no  limit  to  the  number  of  screens  that  may 
be  combined.  The  author  has  at  times  found  it 
necessary  to  use  as  many  as  five  dyes  in  combination 
to  obtain  the  desired  results.  In  Fig.  19,  p  to  u 
inclusive  show  the  rays  in  the  quartz  mercury  arc 
radiation  transmitted  by  six  different  yellow  screens 
between  glass  plates.  These  screens  appear  of  about 
the  same  color  and  transparency  in  daylight.  All 
are  seen  to  transmit  the  yellow  and  green  mercury 
lines,  but  three  of  them  also  transmit  ultra-violet  rays. 
Data  regarding  other  media  and  the  relative  exposures 
and  transparencies  to  tungsten  light  have  been  pre- 
sented elsewhere.10  The  original  negatives  are  of 
course  more  satisfactory  than  the  prints,  because  some 
of  the  fine  detail  is  unavoidably  lost  in  reproduction. 
These  few  specimen  spectrograms  have  been  inserted, 
not  only  on  account  of  the  interest  in  these  special 
cases,  but  also  as  a  means  of  giving  some  idea  of 
the  various  details  to  be  considered  in  the  examina- 
tion and  production  of  special  screens  to  those  who 
may  not  be  familiar  with  the  procedure. 

REFERENCES 

1.  Phil.  Mag.  12,  p.  81. 

2.  Recent  Researches,  p.  47. 

3.  Trans.  Roy.  Soc.  A,  203,  p.  385. 

4.  Ann.  d.  Phys.  IV,  1908,  25,  p.  377. 

5.  Nature,  Nov.  29,  1894,  p.  113. 

6.  Color,  p.  194. 

7.  Ann.  d.  Phys.  1895,  54,  p.  193. 

8.  Carnegie  Inst.  of  Washington. 

9.  Atlas  of  Absorption  Spectra. 
10.  Trans.  I.  E.  S.  9,  p.  472. 


CHAPTER  III 
COLOR-MIXTURE 

17.  That  there  is  a  tremendous  variety  of  colors 
present  in   Nature   can   hardly   escape   the   most  in- 
different  observer.     A  glance   at  a   modern  painting 
reveals  the  same  abundance  of  tints  and  shades  of 
color  created  by  the  hand  of  the  artist  from  a  few 
well-chosen    fundamental    colors.     The    artist    mixes 
colors    in    a     qualitative     manner.      He     sometimes 
begins  painting  with  some  knowledge  of  the  science 
of  color-mixture,  but  after  all  his  knowledge  of  mixing 
colors  is    largely  qualitative    and    based    upon    asso- 
ciation with  his  stock  of  pigments  rather  than  upon  a 
knowledge  of  quantitative  mixture  of  spectral  colors. 
His  success  lies  largely  in  a  thorough  acquaintance 
with  the  tools  at  his  disposal,  which  are  his  pigments, 
yet  an  acquaintance  with  the  science  of  color  is  of 
incalculable  value  to  him,  for  the  experimental  results 
of  the  scientific  study  of  color-mixture  have  largely 
formed    the   foundation   of   pure   and   applied   art   as 
well  as  of  modern  color  theories. 

18.  Subtractive    Method. --There    are    two    dis- 
tinct  methods   of  mixing   colors;    by  addition  and   by 
subtraction  of  light  rays.      In  a   sense,   color,   as   we 
ordinarily  encounter  it,  is_groduced  primarily^by  sub- 
traction (#12).     That  is,  afabricTappears  colored  as 
a  rulebecause  the  chemical  used  in  staining  it  has 
the  property   of    absorbing   certain   visible   rays    and 
of    reflecting    (or    transmitting)    the   remaining   rays. 
This    subtraction    of    colored    rays    from    white    light 

54 


- 


Plate  II.    Fig.  20.    The  Subtractive  Method  of  Mixing  Colors 


. 

Plate  II.    Fig.  21.    The  Additive  Method  <>f;  Mixing  Colors 


COLOR-MIXTURE  55 


results  in  the  residual  colored  light.  The  integral 
color  of  the  light  absorbed  is  said  to  be  comple- 
mentary to  the  color  of  the  light  remaining  if  the 
total  light  in  the  beginning  were  white  light,  say  noon 
sunlight.  Of  course  in  the  foregoing  case  the  ab- 
sorbed color  has  disappeared,  so  there  is  no  oppor- 
tunity to  view  the  complementaries.  Any  pair  of 
complementary  colors  can  be  readily  viewed  by  a 
comparatively  simple  apparatus.  By  means  of  a 
prism  the  spectrum  of  sunlight  is  produced  at  some 
point  in  space.  A  portion  of  this  spectrum  can  be 
deflected  from  the  original  path  by  means  of  a  prism 
of  slight  angle.  The  rays  in  each  beam  can  be  com- 
bined upon  adjacent  spots  of  a  white  surface  by 
means  of  lenses,  with  the  result  that  instead  of  a 
spot  of  white  light  two  adjacent  spots  of  colored  light 
are  seen.  These  two  colored  lights  are  obviously 
complementary,  for  if  they  are  made  to  overlap  they 
will  be  found  to  produce,  by  addition,  a  white  light. 
By  separating  various  portions  of  the  spectrum  all 
the  pairs  of  complementary  colors  are  readily  pre- 
sented to  view.  As  will  be  shown  later,  white  light 
can  be  matched  by  mixing  certain  pairs  of  (and  also 
by  mixing  three  or  more)  spectral  colors.  This  can 
readily  be  demonstrated  by  means  of  variable  slits 
cut  in  a  cardboard  screen  and  held  in  front  of  the 
spectrum.  If  the  slits  have  been  placed  in  their 
proper  position  in  the  spectrum  and  properly  adjusted 
in  width,  white  light  will  result  when  the  rays  from 
these  slits  are  combined  on  a  white  screen  by  means 
of  a  lens. 

The  subtractive-  primary  colors  have  been  termed 
red,  yellow,  and  blue.  In  reality  they  would  be  more 
exactly  described  as  purple,  yellow,  and  blue-green. 
They  are  the  complementaries  of  the  additive  pri- 


56  COLOR  AND   ITS  APPLICATIONS 

maries,  as  will  be  seen  later.  Some  may  prefer  to  use 
the  term  'pink'  or  'magenta'  instead  of  'purple',  but 
the  hue  is  a  purple  consisting  of  red  and  blue.  The 
tri-color  processes  of  printing  and  color  photography 
are  based  upon  the  subtractive  principle  of  mixing 
colors. 

The  principle  of  the  subtractive  method  is  well 
demonstrated  by  Fig.  20  (Plate  II).  If  the  three 
subtractive  primaries,  purple,  yellow,  and  blue-green, 
are  carefully  made  by  the  use  of  transparent  media, 
water  colors  or  printing  inks,  and  are  superposed, 
the  results  shown  in  Fig.  20  are  obtained.  First 
let  us  take  a  simple  case  of  a  yellow  pigment  on  a 
white  surface.  The  light  passes  through  the  colored 
film  and  is  reflected  back  through  it  by  the  white 
surface.  As  the  light  passes  through  the  yellow  pig- 
ment it  is  robbed  of  the  violet  and  blue  rays,  there- 
fore the  light  which  reaches  the  eye  is  white  minus 
violet  and  blue  rays,  and  produces  a  sensation  of 
yellow.  In  the  processes  of  painting  and  color  print- 
ing the  three  disks  may  be  assumed  to  be  micro- 
scopic in  size,  each  being  a  minute  flake  of  pigment. 
If  two  flakes  be  superposed,  a  yellow  above  a  blue- 
green,  a  green  color  is  obtained.  The  yellow  flake 
does  not  transmit  blue  rays,  therefore  the  green  rays 
are  the  only  remaining  rays  that  will  be  transmitted 
by  the  blue-green  pigment.  Ths.se  will  be  reflected 
by  the  white  surface,  and  will  pass  again  through  the 
blue-green  and  yellow  pigments,  undergoing  further 
changes  tending  to  purify  them,  so  that  only  green  rays 
reach  the  eye.  If  the  blue-green  flake  is  above  the 
yellow  flake,  the  explanation  must  be  reversed,  but 
with  the  same  result.  The  blue-green  flake  trans- 
mits blue  and  green  rays;  however,  the  yellow  flake 
does  not  transmit  blue  rays.  Therefore,  only  the 


COLOR-MIXTURE  57 


green  rays  will  eventually  be  reflected  to  the  eye. 
In  the  same  manner  the  blue  of  the  purple  is  sub- 
tracted by  the  yellow  flake,  and  as  purple  consists  of 
red  and  blue  rays  only,  the  red  rays  remain  to  be 
reflected  to  the  eye.  Therefore,  yellow  and  purple 
flakes  superposed  produce  red.  Likewise  the  blue- 
green  flake  does  not  transmit  red  light,  so  that  super- 
position of  blue-green  and  purple  flakes  results  in 
blue  light  being  reflected  to  the  eye.  It  is  further 
seen  that  the  superposition  of  the  three  subtrac- 
tive  primaries  results  in  a  total  extinction  of  light 
and  black  is  the  result.  For  instance,  where  the 
yellow  and  purple  disks  overlap,  red  results.  The 
blue-green  disk  does  not  transmit  red  rays,  so  where 
it  overlaps  the  red  disk  a  total  extinction  results. 

Much  interesting  information  may  be  obtained 
by  carefully  studying  Fig.  20.  Strips  of  colored 
gelatine  laid  over  each  other  in  checkerboard  fashion 
present  many  striking  examples  of  the  subtractive 
method  of  mixing  colors.  In  ordinary  artificial  light, 
screens  made  of  ethyl  violet  (purple),  uranin  or 
aniline  yellow,  and  filter  blue-green,  are  excellent 
dyes  for  making  the  subtractive  primaries  for  demon- 
strating the  foregoing  by  superposition.  Ethyl  violet 
and  naphthol  green  are  practically  complementary,  so 
that  when  superposed  no  light  rays  are  transmitted. 

19.  Additive  Method. — -As  already  indicated, 
there  are  two  distinct  methods  of  mixing  color, - 
the  additive  and  subtractive,  —  but  close  investiga- 
tion often  reveals  both  processes  entering  into  some 
part  of  the  production  of  color.  The  additive  method 
always  tends  toward  the  production  of  white,  whereas 
the  subtractive  method  tends  toward  the  production 
of  black.  The  additive  primaries  are  red,  green,  and 
blue.  Some  prefer  to  use  the  term  *  violet*  instead 


58  COLOR  AND  ITS  APPLICATIONS 

of  'blue.'  Blue,  however,  appears  satisfactory  and  is 
a  safer  term  than  violet,  because  there  are  a  great 
many  who  apply  the  term  violet  to  purples. 

Long  ago  it  was  demonstrated  that,  by  proper 
mixtures  of  the  three  well-chosen  primary  colors, 
any  color  can  be  matched.  This  is  largely  due  to 
the  fact  that  the  eye  is  a  synthetic  rather  than  an 
analytic  instrument.  In  Fig.  21  (Plate  II)  are  illus- 
trated the  principles  of  color-mixture  by  the  additive 
method.  It  is  seen  that  red  added  to  green  produces 
yellow;  and  further,  when  blue  is  added  to  this  com- 
bination white  is  produced.  In  other  words,  yellow 
and  blue  mixed  by  addition  produce  white.  It  is  well 
known,  however,  that  yellow  and  blue  (in  reality 
blue-green)  pigments  when  mixed  by  the  subtractive 
method,  as  is  done  in  painting  and  color  printing, 
produce  green.  This  is  a  much  confused  point,  but 
is  very  simply  explained  when  the  character  of  the 
procedure  of  mixture  is  analyzed.  Red  and  blue 
when  added  produce  purple;  and  blue  and  green 
produce  blue-green.  It  is  to  be  noted  that  combina- 
tions of  two  of  the  additive  primaries  produce  the 
subtractive  primaries  and  vice  versa.  The  additive 
method  can  be  readily  demonstrated  by  the  use  of 
colored  lights  projected  upon  a  white  surface.  Prop- 
erly selected  color-screens  are  necessary,  but  can  be 
readily  made  from  aniline  dyes  by  carefully  mixing 
them.  It  is  difficult  to  describe  the  procedure  quan- 
titatively, but  there  is  no  difficulty  in  producing  the 
proper  colors. 

Owing  to  the  very  unsatisfactory  state  of  color 
terminology,  it  is  impossible  to  present  an  accurate 
and  definite  list  of  complementary  hues.  However, 
a  few  complementaries  are  given  in  Table  IV. 


COLOR-MIXTURE 


59 


TABLE  IV 
Complementary  Hues 

1  Red .Blue-green  (Cyan  blue) 

Orange-red Green-blue  (bluish 

Orange Blue 

I  Yellow .*.  Blue-violet 

Yellow-green Violet-purple 

<  Green * .  Purple  (magenta)       £ 

Wave-length  of  Complementary  Spectral  Hues 


0.6562  p 

0.4921  ft 

.5671  fi 

.4645 

.6077 

.4897 

.5644 

.4618 

.5853 

.4854 

.5636 

.4330 

.5739 

.4821 

0 


An  excellent  scheme  for  showing  the  comple- 
mentaries  is  to  arrange  the  spectrum  around  the 
circumference  of  a  circle  filling  a  gap  between  the 
ends  of  the  spectrum,  violet  and  red,  with  a  series 
of  purples  from  bluish  purple  to  reddish  purple.  This 
has  been  called  a  color  wheel,  and  is  diagrammatically 
shown  in  Fig.  22.  Here 
yellow  and  violet  are  shown 
as  complementary.  This  may 
appear  inconsistent  with  the 
foregoing  discussion,  but  it 
will  be  noted  that  the  terms 
4  blue'  and  *  violet'  (as  well 
as  other  color  names)  are 
indefinite.  The  term  'blue' 
will  always  mean  a  spectral 
blue,  but  when  used  as  a 
primary  color  its  hue  is  defi- 
nite, whether  the  term  stands 
for  blue,  violet,  or  blue-violet, 
have  been  correctly  applied 


Fig.  22.  —The  color-wheel  for  show- 
ing complementary  hues. 


If  the  complementaries 
to   the  color  wheel,  a 

neutral  gray  should  be  obtained  when  it  is  rapidly 

rotated. 


60  COLOR  AND   ITS  APPLICATIONS 

20.  Juxtapositional     Method. — If     a     color     be 
broken  up  into  its  component  colors  and  the  latter  be 
applied  in  small  dots  with  the  point  of  a  brush,  the 
sensation  of  the  original  color  will  be  obtained  if  it 
be  viewed  from  a  distance  at  which  the  eye  is  un- 
able to  resolve  the  individual  dots  and  providing  the 
relative    areas   covered   by  the   various   colored   dots 
are   correctly  balanced.     Colors,  excepting   those  en- 
countered in  the  spectrum,  are  usually  far  from  mono- 
chromatic  (#12),    (Figs.    122,    123).     For   instance,   a 
colored  fabric  which  may  appear  a  pure  red  will  be 
found  to  reflect  rays  throughout  considerable  range 
of  wave-lengths.    If  these  component  colors  be  repre- 
sented as  pure  as  possible  in  minute  dots  of  proper 
relative    amounts,    the    foregoing    result    is    readily 
obtained.     For  instance,  if  one  end  of  a  pack  of  cards 
be  painted  red  and  the  other  end  green,  on  revers- 
ing every  other  card  and  viewing  an  end  of  the  pack 
at  a  distance  of  several  feet,  it  will  appear  yellow  in 
color.      The   brightness   apart  from   hue   will    be    an 
average    brightness.      Many  interesting    experiments 
can   be   performed   by  ruling   alternate   fine   lines   of 
different  colors  on  paper  or  on  glass.     For  instance, 
purple  and  green  lines  alternated  on  paper  will,  if 
well  chosen,  produce  an  appearance  of  gray  at  some 
distance.     Such   a   method   of  breaking   a   composite 
color   into    more    nearly    monochromatic    components 
and  applying  the  latter  in  the  form  of  minute   dots 
is  the  foundation  of  the  principle  of  impressionistic 
painting.     The  processes  of  color  photography  devised 
by  Joly,  Lumiere  and  others  are  also  based  on  this 
principle. 

21.  Simple     Apparatus    for     Mixing     Colors.  - 
There    are   very    elaborate    color-mixing   instruments 
on  the  market  for  the  purpose  of  demonstrating  the 


COLOR-MIXTURE  61 


theory  and  practise  of  color-mixture.  Apparatus  that 
deals  with  spectral  colors  is  as  a  rule  the  most  sat- 
isfactory for  accurate  study  and  demonstration.  How- 
ever, inasmuch  as  the  colors  ordinarily  available  in 
practise  are  far  from  monochromatic,  that  is,  far  from 
spectral  purity,  there  is  much  virtue  in  the  simpler 
forms  of  apparatus  that  can  be  made  at  small  expense. 
In  fact,  for  the  foregoing  reason  the  results  obtained 
with  some  of  the  simpler  instruments  for  demon- 
stration are  more  readily  interpreted  and  applicable 
in  practise  than  those  obtained  with  apparatus  dealing 
with  pure  spectral  colors. 

Maxwell's  disks  offer  a  ready  means  for  mixing 
colors.  A  shaft  is  arranged  so  as  to  be  revolved  at 
high  speed.  Colors  painted  on  a  disk  can  thus  be 
mixed  by  rotating  it  at  a  high  speed  owing  to  per- 
sistence of  vision.  Light  sensations  do  not  reach 
their  full  value  immediately  upon  application  of  the 
stimulus,  nor  do  they  decay  to  zero  immediately  upon 
the  cessation  of  the  stimulus.  An  infinite  number  of 
mixtures  of  pigments,  including  black  and  white, 
can  be  made  with  such  a  simple  disk.  Colored 
papers  cut  in  circles  and  slit  along  one  of  the  radii 
can  thus  be  overlapped  to  any  degree,  and  by  the 
use  of  circles  of  various  sizes  a  number  of  mixtures 
can  be  produced  upon  the  same  disk.  This  method 
is  not  truly  an  additive  one,  excepting  in  the  addition 
of  hues.  The  brightness  is  the  mean  of  the  sepa- 
rate brightnesses,  each  weighted  by  its  angular 
extent.  In  Fig.  23  are  typical  color  disks  for  mixing 
colors  to  produce  grays.  In  I  and  III  are  repre- 
sented pairs  of  complementary  colors  respectively, 
yellow  and  blue,  and  green  and  purple.  The  inner 
circle  consists  of  black  and  white,  which  can  be  varied 
in  angular  amounts  to  produce  a  neutral  gray  to 


62 


COLOR  AND  ITS  APPLICATIONS 


match  the  gray  produced  by  the  addition  of  the  two 
hues.  In  II  are  represented  the  three  primary  colors 
which  when  mixed  by  rotation  produce  a  neutral 
gray  which  is  readily  matched  by  means  of  the  inner 
black  and  white  disks.  These  matches  made  under 
one  illuminant  will  not  ordinarily  remain  matches 
under  another  illuminant.  Much  of  the  early  work 
in  the  science  of  color  was  done  by  means  of  rotating 
disks  and  even  today  they  are  extremely  valuable 
in  some  investigations.  The  disks  represented  in 
Fig.  23  can  be  readily  made  from  Zimmerman's 


IL 

Fig.  23.  — Maxwell  disks. 

colored  papers.  These  papers  are  indicated  in  the 
catalogue  by  the  letters  of  the  alphabet  and  are  given 
herewith  as  used  in  the  disks  already  described. 
Yellow  is  designated  as  g,  blue  as  o,  green  as  i,  red 
as  by  and  purple  as  a.  For  the  black  and  white  sectors 
any  neutral  tint  papers  with  dull  finish  are  satis- 
factory for  producing  the  grays. 

The  additive  and  subtractive  methods  as  illus- 
trated in  Figs.  20  and  21  (Plate  II)  can  readily  be 
demonstrated  in  permanent  charts.  The  imported 
colored  papers  have  been  found  satisfactory,  owing 
to  their  comparative  purity  and  unglazed  surfaces. 
For  demonstrating  the  subtractive  method  by  the 
three  overlapping  disks  the  six  colors  and  black  are 
surrounded  with  a  white  background.  The  Zimmer- 


COLOR-MIXTURE  63 


man  colors,  designated  by  a,  #,  Z,  &,  z,  and  o,  may  be 
used  respectively  for  purple,  yellow,  blue-green,  red, 
green,  and  blue.  These  six  colors,  with  black  and 
white,  are  sufficient  for  the  construction  of  charts  for 
the  additive  and  subtractive  methods.  For  demon- 
strating the  additive  method  the  three  disks  should 
be  surrounded  with  black  background,  but  in  the 
case  of  the  subtractive  method  the  background  should 
be  white.  For  demonstrating  these  two  methods 
of  color-mixture  with  artificial  light  by  means  of 
transparent  media,  purple  and  green  are  readily 
produced  by  using  gelatines  dyed  with  ethyl  violet 
and  naphthol  green  respectively.  When  these  two 
colored  gelatines  are  superposed  in  proper  densities 
of  coloring,  no  light  is  transmitted.  When  light  is 
passed  through  these  media  in  juxtaposition  in  proper 
relative  amounts  and  combined  on  a  neutral  tint 
diffusing  surface  a  white  light  is  produced.  These 
two  colors  afford  an  excellent  example  of  comple- 
mentary colors  when  used  with  artificial  light.  In 
daylight  the  ethyl  violet  screen  appears  deep  blue  in 
color,  instead  of  appearing  purple  as  it  does  in  the 
light  from  a  tungsten  incandescent  lamp.  Other 
transparent  media  for  further  demonstrating  these 
methods  are  readily  selected  from  the  many  organic 
dyes  available.  Uranin,  fluorescein,  carmine,  patent 
blue,  and  filter  blue-green  are  satisfactory. 

In  Fig.  24  the  construction  of  an  erratic  color- 
mixing  disk  is  illustrated.  To  a  disk  of  stiff  card- 
board a  sectored  disk  of  cardboard  is  rigidly  fastened 
by  means  of  a  circular  rivet.  The  latter  disk  has 
two  60°  openings,  as  shown  in  a.  Another  disk, 
arranged  concentric  with  the  other  disks  and  between 
them,  is  permitted  to  slip  at  will  about  the  rivet  as 
an  axis.  If  the  latter  disk  is  prepared  as  shown  in 


64 


COLOR  AND   ITS  APPLICATIONS 


b  many  striking  colors  are  obtained  on  rotating  the 
combination. 

Fig.  25  illustrates  a  simple  arrangement  for  color- 
mixing.  The  wheel  is  similar  to  that  employed  in  the 
Simmance-Abady  flicker  photometer.  The  periphery 


Fig.  24.  —  An  erratic  color-mixing  disk. 

of  this  wheel  consists  of  truncated  cones  pointing 
in  opposite  directions.  The  axes  of  the  cones 
are  eccentrically  placed  at  equal  distances  on  either 
side  of  the  axis  of  the  wheel  and  parallel  to  it. 
In  the  angular  position  shown  in  the  illustration, 


# 


L' 
* 


Fig.  25. — A  simple  color-mixer. 

the  eye,  looking  at  the  wheel  in  a  direction  at  right 
angles  to  the  axis,  sees  one  conical  surface  illumi- 
nated by  one  light,  Ly  and  the  other  by  the  other  light, 
Lf.  Colored  screens,  SS,  are  interposed  between 
the  wheel  and  the  light  sources.  By  having  the  lamps 
movable  on  a  track  any  combination  of  brightnesses 
of  two  colors  from  transmitting  media  can  be  mixed 


COLOR-MIXTURE 


65 


by  rotating   the   wheel  rapidly.     Pigments   may   also 
be  applied  directly  to  the  wheel. 

In  Fig.  26  is  illustrated 
another  simple  instrument 
for  mixing  the  colors  from 
either    opaque 
parent  media. 


or    trans- 
A  wooden 

box  is  constructed  as 
shown  and  painted  black 
inside.  G  is  a  transparent 
plate  glass  and  OO  are 
ground  opal  glasses  free 
from  color.  In  mixing  the 
colors  of  two  transparent 
media,  CC,  the  lamp,  L, 
is  moved  to  and  fro  on  its 
track.  Thus  any  propor- 
tions of  the  two  colors  can 
be  mixed.  If  the  colors 
of  two  opaque  substances 


Fig.   26.  —  A  simple   color-mixer   for 
transparent  or  opaque  media. 


are    to    be    mixed,    CC 


and    OO    are    removed,    the 


/ 

/\ 


Eye 


\ 


\ 


\ 


\ 


C/ 


\ 


Fig.  27.  —  Lambert's  color-mixer. 


colored  objects  are 
placed  at  PP,  and  the 
lamp  is  moved  to  and 
fro  as  before.  The 
range  of  mixtures  in 
the  last  case  is  not 
infinite  as  in  the  case 
of  transparent  media; 
however,  modifica- 
tions can  readily  be 
iade  so  that 
ie  range  is 
extended. 


A  simple  experiment  devised  by  Lambert,  though 
not  having  the  flexibility  of  the  foregoing  instruments, 


/ 


66 


COLOR  AND   ITS  APPLICATIONS 


is  of  interest  owing  to  its  extreme  simplicity.  It  is 
illustrated  in  Fig.  27.  G  is  a  plate  glass  and  CC  are 
colored  objects.  The  colors  are  mixed  one  by  re- 
flection, the  other  by  transmission.  By  turning  the 
glass  and  shifting  the  eye  the  proportions  can  be 
altered  considerably. 

An  apparatus  of  considerable  use  is  a  booth  con- 
taining red,  green,  and  blue  incandescent  lamps  con- 
trolled by  rheostats.  If  the  colors  are  carefully  made 
many  interesting  experiments  can  be  performed,  in- 
cluding the  effect  of  quality  or  spectral  character  of 
light  upon  colored  objects  (#67). 

Many  instructive  experiments  can  be  produced 
by  the  use  of  shadows  cast  by  colored  lights.  One  of 

especial  interest  is  shown 
in  Fig.  28,  because  it  pre- 
sents the  additive  prima- 
ries, their  complementaries 
(the  subtractive  primaries) 
and  white  light  produced 
by  the  sum  of  the  three 
primary  colors,  red, 
green,  and  blue.  In  the 
middle  of  a  circle  of  white 
diffusing  blotting  paper 
stiffened  by  a  board  are 
erected  three  planes  of 
white  diffusing  material  about  eight  inches  in  height. 
The  latter  meet  at  the  center  of  the  circle  at 
angles  of  120  degrees  with  each  other.  At  points 
several  feet  away,  along  the  three  arrows,  red,  green, 
and  blue  lights  are  placed  somewhat  above  the  plane 
of  the  circle.  These  should  be  small  sources  and 
quite  powerful,  concentrated  tungsten  filament  lamps 
being  quite  satisfactory.  The.  experiment  is  best 


Fig.  28. — A  shadow  demonstration  of 
the  additive  and  subtractive  methods 
of  color-mixture. 


COLOR-MIXTURE  67 


seen  if  the  plane  of  the  circle  is  vertical.  It  will  be 
seen  that  the  nearly  rhombic  areas  on  the  circle 
indicated  by  #,  G,  and  B  each  receive  light  from  only 
one  source.  These  areas  will  then  appear  respectively 
red,  green,  and  blue.  The  areas  on  the  opposite 
sides  of  the  circle,  Z?G,  P,  and  7,  each  receive  light 
from  only  two  sources.  They  appear  in  colors  com- 
plementary to  the  above  primaries.  They  also  rep- 
resent the  subtractive  primaries  and  the  colors  which 
remain  after  red,  green  and  blue  are  subtracted 
respectively  from  three  white  lights.  The  remaining 
areas  of  the  circle  marked  W  represent  the  regions 
which  receive  light  from  each  of  the  three  sources, 
with  the  result  that  if  the  colors  and  intensities  of 
the  light  sources  are  correct,  and  if  the  sources  are 
sufficiently  distant  in  comparison  with  the  size  of 
the  circle,  these  areas  appear  a  uniform  white.  This 
experiment  is  simple  and  is  very  satisfactory  for  dem- 
onstration before  large  audiences.  The  lights  should 
be  controlled  by  separate  switches  and  rheostats. 

A  rotating  disk  can  be  readily  colored,  so  that  it 
will  appear,  when  viewed  through  a  radial  slit  placed 
close  to  it,  a  fair  approximation  to  the  spectrum.  The 
mixing  of  the  colors  by  rotation  obviates  the  neces- 
sity of  the  great  care  in  blending  colors  in  painting 
a  spectrum  that  is  to  be  viewed  when  stationary. 
The  colors  will  not  be  of  spectral  purity,  owing  to 
the  limitations  of  the  pigments,  but  the  disk  will  be 
instructive  and  affords  a  ready  means  of  producing 
a  spectrum  for  reproduction  by  color  photography. 
An  approximation  to  the  prismatic  spectrum  can  be 
readily  produced  as  shown  in  Fig.  29.  The  approxi- 
mation can  be  made  as  close  as  desired  by  touching 
up  various  points  with  pigments  where  necessary 
or  by  varying  the  geometric  figures.  If  a  circle  be  cir- 


68 


COLOR  AND  ITS  APPLICATIONS 


cumscribed  about  the  inner  square  and  a  square  in 
turn   be   circumscribed   about   this   circle,  and  so  on 

until  four  circles  and 
four  squares  are  pro- 
duced, the  skeleton  of 
the  figure  is  ready  for 
coloring.  If  the  inner 
square  be  painted 
black  and  the  succeed- 
ing hollow  squares  be 
painted  red,  yellow, 
green,  and  violet  re- 
spectively and  the  re- 
mainder of  the  outer 
circle  be  Dainted  black, 

Fig.  29.  —  Illustrating  a  disk  for  approxima-  £   .  . 

ting  a  prismatic  spectrum.  &     tair     approximation 

to  a  prismatic  spec- 
trum is  obtained.  The  same  results  can  be  pro- 
duced with  more  difficulty  with  only  three  colors 
-red,  green,  and  blue.  If  the  spectrum  pro- 
duced by  rotation  is  not  satisfactory  at  all  points,  it 
can  be  readily  made  so  by  the  judicious  use  of 
pigments,  or,  as  already  stated,  by  altering  the  geo- 
metric figures. 

The  more  simple  methods  of  mixing  colors  have 
been  described  in  this  chapter;  however,  it  will  be 
borne  in  mind  that  many  of  the  instruments  and 
methods  considered  in  succeeding  chapters  are  di- 
rectly or  indirectly  applicable  to  color-mixture. 


REFERENCES 

Captain  W.  de  W.  Abney,  Colour  Measurements  and  Mixture, 
London,  1891. 

O.  N.  Rood,  Colour,  1904. 

Chevreul,  Harmony  and  Contrast  of  Colours,  1839. 


v 
CHAPTER   IV 

COLOR  TERMINOLOGY 

22.  Hue,  Saturation,  and  Brightness. —  One  of  the 
greatest  needs  in  the  art  and  science  of  color  is  a 
standardization  of  the  terms  used  in  describing  the 
quality  of  colors  and  an  accurate  system  of  color 
notation.  The  term  *  color'  in  its  general  sense,  is 
really  synonymous  to  the  term  'light.'  It  is  used  here 
by  preference  because  it  implies  the  consideration 
of  the  appearance  of  a  surface  or  material  object. 
The  spectrophotometer  is  the  most  analytic  instru- 
ment for  examining  colors  (#26).  By  means  of  it  the 
amounts  of  light  of  all  wave-lengths  reflected  (or 
transmitted)  by  a  colored  medium  may  be  obtained. 
These  data  are  plotted  in  the  form  of  curves  shown 
by  the  dashed  lines  in  Fig.  12.  The  full  line  curves 
represent  the  reflection  (or  transmission)  coefficients 
of  the  pigments  for  energy  for  various  wave-lengths. 
If  the  region  under  one  of  the  curves  indicated  by  a 
dashed  line  be  integrated  and  this  area  compared 
with  that  obtained  with  the  same  illuminant  for 
a  white  diffusing  surface  of  known  total  reflecting 
power,  the  relative  brightness  of  the  colored  medium 
under  this  illumination  is  obtainable.  The  domi- 
nant hue  which  is  discussed  below  may  be  usually 
approximately  determined  by  inspection  of  the  curve, 
although  in  many  cases  it  is  impossible  to  estimate 
the  dominant  hue  in  this  manner.  It  is  thus  seen  that 
although  the  spectrophotometer  is  a  valuable  instru- 
ment for  analyzing  colors,  there  are  further  require- 

69 


70  COLOR  AND  ITS  APPLICATIONS 

ments  in  color  work  better  met  by  other  instruments 
(Chapter  V). 

The  quality  of  any  color  can  be  accurately  de- 
scribed by  determining  its  hue,  saturation  or  purity, 
and  its  brightness.  (The  latter  term  is  analogous 
to  the  term  *  value'  as  used  by  the  artist.)  In  the 
broadest  sense,  white,  gray,  and  black  are  here  con- 
sidered as  colors,  and  a  mere  change  in  brightness 
alone  is  considered  as  a  change  in  color.  It  appears 
necessary  to  assume  this  broad  definition  of  color, 
inasmuch  as  brightness  is  distinctly  one  of  the 
products  of  color  analysis.  Hue  is  suggested  in 
the  name  applied  to  the  color.  The  dominant  hues 
of  most  colors  are  accurately  represented  by  spectral 
colors ;  however,  there  are  composite  colors,  —  the 
purples,  which  consist  of  red  and  violet,  for  which  no 
spectral  colors  are  found  to  represent  their  hues.  In 
these  cases  it  is  satisfactory  to  determine  the  domi- 
nant hue  of  the  complementary  colors.  The  satu- 
ration or  purity  is  a  measure  of  the  relative  amount 
of  white  light  in  the  color.  In  other  words,  all  colors 
excepting  purples  can  be  matched  by  diluting  spectral 
light  of  a  definite  wave-length  with  white  light.  The 
greater  the  percentage  of  white  light  required  in  the 
mixtures,  the  less  saturated  the  colors  are  said  to 
be.  The  brightness  of  a  color  can  be  found  by  com- 
paring it  by  means  of  a  photometer  with  a  surface 
of  known  brightness.  It  is  well  to  note  that  in  the 
analysis  of  a  color  its  absolute  brightness  is  measured 
by  comparing  it  with  a  brightness  of  known  value. 
Inasmuch  as  its  brightness  depends  upon  the  inten- 
sity of  illumination  of  a  given  spectral  character,  its 
reflection  coefficient  for  a  standard  white  light  should 
be  determined  in  order  to  compare  it  with  other 
colors  in  this  respect.  This  latter  measurement  in- 


COLOR  TERMINOLOGY 


71 


volves  all  the  difficulties  of  color-photometry  treated 
in  Chapter  IX. 

There  is  much  confusion  in  the  application  of  the 
terms  'tint,'  'tone,'  'shade,'  'intensity,'  etc.  Many 
use  these  terms  wholly  unjustifiably.  It  is  true  that 
the  final  usage  is  somewhat  a  matter  of  choice  at  the 
present  time,  but  the  terminology  adopted  here  appears 
to  the  author  to  be  consistent  with  other  nomen- 
clature adopted  by  the  physicist,  photometrist,  and 


Fig.  30.  —  Disk  'a,'  for  varying  only  the  saturation  of  a  color. 
—  Disk  « b,'  for  varying  only  the  brightness  of  a  color. 

lighting  expert,  and  best  justified  by  usage  and  the 
dictates  of  common  sense.  On  diluting  a  color  with 
white  light,  tints  are  obtained;  that  is,  tints  are  un- 
saturated  colors.  By  the  admixture  of  black  to  a 
color  (in  effect  the  same  as  reducing  the  intensity  of 
illumination)  the  brightness  is  diminished  without 
altering  either  the  hue  or  the  saturation,  and  various 
shades  are  produced.  Only  the  relative  brightnesses 
of  shades  are  usually  of  interest,  although  for  obtain- 
ing a  basis  of  notation  it  may  be  desirable  to  deter- 
mine their  absolute  values.  In  a,  Fig.  30,  is  shown  a 
simple  means  of  varying  the  saturation  of  a  color 
without  altering  either  the  hue  or  brightness.  On  a 


72  COLOR  AND   ITS  APPLICATIONS 

circle  of  colored  paper  is  glued  a  gray  paper  of  the 
same  brightness  for  the  given  illumination  and  of 
the  form  shown  by  the  shaded  area.  On  rotating 
the  disk  this  gray  will  be  mixed  in  various  angular 
proportions  from  360  deg.  to  0  deg.  The  gray  paper, 
having  been  selected  of  the  same  brightness  as  the 
colored  paper  under  the  illuminant  used  in  the  experi- 
ment, does  not  alter  the  brightness  upon  mixing 
the  two  components  by  rotation;  being  non-selective 
in  its  reflection  it  does  not  alter  the  hue.  Thus 
various  degrees  of  saturation  of  the  original  color 
are  obtained. 

The  brightness  can  be  varied,  as  shown  in  6,  Fig. 
30,  without  altering  either  the  hue  or  saturation  by 
fastening  to  the  original  circle  of  colored  paper  a 
black  paper  cut  in  the  same .  form  as  the  gray  paper 
shown  in  a.  If  the  paper  were  perfectly  black  it  is 
seen  that  it  cannot  alter  either  the  hue  or  satura- 
tion. As  a  matter  of  fact  no  available  black  papers 
are  totally  non-reflecting,  so  that  some  light  is  added 
to  the  color.  This  can  be  reduced  to  a  minimum, 
however,  by  the  use  of  a  hole  in  a  deep  velvet-lined 
box.  In  this  case  the  black  sectors  shown  in  b  would 
be  replaced  by  openings  of  the  same  contour  in  the 
disk.  For  convenience  of  construction  the  areas 
occupied  by  the  black  and  colored  papers  may  be 
reversed.  In  this  connection  it  is  well  to  emphasize 
that  ordinary  black  surfaces  are  far  from  totally 
absorbing.  This  can  readily  be  demonstrated  by 
a  box  open  at  one  end  lined  with  black  velvet.  Over 
the  open  end  place  a  black  cardboard  with  an  opening  in 
it  and  it  will  be  seen  that  the  opening  will  appear  very 
much  darker  than  the  black  surface  surrounding  it.  The 
foregoing  demonstration  may  be  easily  performed  by 
varying  the  brightness  of  a  colored  paper  relative  to  that 


COLOR  TERMINOLOGY 


73 


of  a  paper  of  the  same  color  which  surrounds  it,  by  vary- 
ing the  intensity  of  the  illumination  of  the  patch  at  the 
same  time  maintaining  the  absolute  brightness  of  the 
surroundings  constant. 

Instruments  have  been  designed  for  the  analysis  of 
color  quality  into  the  three  component  factors,  hue, 
saturation,  and  brightness.  These  are  treated  hi  #27. 

23.  Tri-color  Method.  —  It  is  well  known  that 
any  color  can  be  matched  by  combining  the  three 
primary  colors,  red,  green,  and  blue,  in  proper  pro- 
portions. Many  instruments  have  been  devised  for 
this  purpose,  the  most  elementary  being  the  Maxwell 
disks,  and  the  more  elaborate  and  accurate  are  those 
employing  spectral  colors.  The  results  of  such  a 
method  are  expressed  mathematically  in  the  equation 
xR  +  yG  +  zB=  C,  where  the  values  of  x,  y,  and  z 
are  the  fractional  parts  of  the  red,  green,  and  blue 
lights,  respectively,  that  must  be  combined  to  match 
the  color,  C.  This  method  has  limitations  because 
it  does  not  give  the  results  directly  in  terms  of  hue, 
saturation,  and  bright- 
ness. Some  of  these  in- 
struments, however,  can 
readily  be  adapted  to  the 
measurement  of  the  last 
two  factors.  In  order  to 
plot  the  values  of  jc,  z/, 
and  z,  it  is  necessary  to 
employ  tri-linear  coordi- 
nates, there  being  three 
variables  to  be  repre- 

sented.    The  results  are     Fig.  3i.-The  Maxwell  coior-triangie. 
readily    represented    in 

the  Maxwell  color  triangle,  illustrated  in  Fig.  31.  The 
green  component  increases  from  zero  at  the  base 


74  .     .    COLOR  AND  ITS  APPLICATIONS 

line,  RB,  to  100  per  cent  at  G.  Likewise  the  red, 
Rj  and  green,  G,  components  increase  from  zero  at 
the  base  of  the  perpendiculars  erected  from  the  sides 
respectively  opposite  to  their  apexes.  The  data  are 
plotted  by  erecting  three  perpendiculars  proportional 
to  the  respective  values  of  /?,  G,  and  J5,  starting  at 
points  in  the  opposite  sides  of  the  triangle  respectively, 
such  that  the  three  perpendiculars  intersect  at  the 
same  point.  Purples  are  found  along  the  base  line, 
RBj  varying  in  the  proportions  of  R  and  B  from  R  =  0, 
B  =  100,  to  R  =  100,  B  =  0.  Yellows  are  found  along 
RG  and  blue-greens  along  GB.  White,  which  is 
usually  represented  by  J/?+^G+JZ?  =  W,  is  found 
at  the  center  of  the  triangle.  The  curved  line  rep- 
resents the  positions  of  the  spectral  colors  in  the 
color  triangle;  that  is,  each  point  on  the  curve  rep- 
resents the  primary  sensation  values  of  a  particular 
spectral  color.  Some  important  lines  of  the  spectra 
of  cadmium  and  mercury  are  also  shown.  The  less 
saturated  colors  are  found  near  the  center  of  the 
triangle  and  the  more  saturated  ones  near  the  sides. 
It  is  thus  seen  that  spectral  colors  throughout  a  large 
range  of  wave-lengths  arouse  the  three  primary 
sensations,  according  to  the  Young-Helmholtz  theory. 
(See  #28,  47.)  The  primaries  of  course  are  found 
at  the  angles  of  the  triangle.  Complementaries  are 
represented  as  being  on  opposite  sides  of  the  center 
of  the  triangle  on  a  straight  line  passing  through  it. 
The  dominant  hue  of  a  color  is  found  by  drawing  a 
straight  line  from  the  center  of  the  triangle  through 
the  point  representing  the  color  and  continuing  it 
until  it  intersects  the  curve  representing  the  spectrum. 
The  latter  point  of  intersection  represents  the  domi- 
nant hue  of  the  color.  The  tri-color  method  in- 
volves the  use  of  an  invariable  white  light,  that  is 


COLOR  TERMINOLOGY 


75 


0.50 


0.60 
WAVE  LENGTH 


Fig.  32.  —  Spectral  complementaries. 


noon  sunlight  or  its  equivalent.     A  curve  represent- 
ing spectral  complementaries  is  shown  in  Fig.  32. 

"Results  obtained  by 
this  general  method  with 
different  instruments  are 
likely  to  vary  considerably. 
This  is  due  in  part  to  vari- 
ations in  the  spectral  char- 
acter of  the  white  light 
standard,  and  also  to  the 
transmission  characteris- 
tics of  the  color-screens 
used  in  these  instruments 
not  employing  spectral 
primary  colors.  The  primary  sensation  values  of  the 
screens  should  be  determined  and  the  measurements 
be  given  in  sensation  values  (#28).  The  use  of  the 
plane  triangle  is  limited  to  the  plotting  of  the  analyses 
of  colors  of  equal  brightness.  In  order  to  include 

the  brightness  factor  the  figure 
takes  the  form  of  a  solid  inverted 
pyramid,  shown  in  Fig.  33.  The 
various  triangular  planes  parallel 
to  the  base  represent  planes  for 
plotting  colors  of  different  bright- 
nesses. The  apex  represents 
black.  A  line  joining  the  point, 
W  (white),  with  the  apex  passes 
through  a  complete  range  of 
shades  of  white,  that  is,  of  grays. 
Along  the  dotted  line  from  x  to 
the  apex  are  a  series  of  colors 
of  constant  hue  and  saturation, 
but  varying  in  brightness.  The  color  pyramid  has  been 


R 


Black 
Fig.  33.  —  A  color  pyramid. 


modified  in  various  rays  to  fit  experimental  results  in- 


76 


COLOR  AND   ITS  APPLICATIONS 


volving  physiological  and  psychological  influences.   One 

of  these  modifications  from  Titchener  l  is  shown  in  Fig. 

34.  At  the  two  poles  of  this  double  pyramid  are  the 

extremes  of  white  and  black; 
upon  the  axis  connecting  the  two 
poles  are  located  the  complete 
range  of  grays.  Around  the  pe- 
riphery of  the  middle  plane  are 
located  those  colors  of  middle 
brightness  and  maximal  saturation. 
Other  points  in  the  solid  represent 
other  colors  of  varying  hue,  satura- 
tion, and  brightness.  From  the  base 
toward  white,  tints  are  found;  in 
the  other  direction  shades  are 
found.  This  pyramid,  it  will  be 
noted,  has  four  sides,  the  four 
angles  representing  red,  yellow, 

Fi^3>- 7he  double  pyra-  green,   and  blue.     Obviously   this 

mid.     (After  Titchener.)  J 

solid  does  not  directly  represent 
color  analyses  as  obtained  by  the  tri-color  method. 
Its  significance  will  be  better  understood  on  referring 
to  the  Hering  theory  of  color  vision  in  #  49. 

The  tri-color  method  is 
discussed  further  in  Chapter 
V.  A  simple  means2  of  dem- 
onstrating the  Maxwell  color 
triangle  in  actual  colors  is 
illustrated  in  Fig.  35.  A  box 
6  inches  in  depth,  and  whose 
section  forms  an  equilateral 
triangle  about  18  inches  on 

.  -  -I*  j         Fig«  36.  —  A  demonstration  color 

a    side,    is    made    of    wood,        triangle. 

with  its  back  containing  vent 

holes.     A    ground    flashed-opal    glass    in    the    form 


COLOR  TERMINOLOGY  77 

of  an  equilateral  triangle  somewhat  smaller  than 
the  section  of  the  box  forms  the  front  side.  In 
the  three  corners  of  the  box  are  placed  respectively, 
red,  green,  and  blue  spherical-bulb,  concentrated- 
filament  tungsten  lamps.  After  proper  adjustments 
of  the  position  and  color  of  the  lamps,  the  diffusing 
glass,  which  has  its  roughed  side  inward,  assumes 
the  colors  of  a  color  triangle.  A  close  approximation 
can  be  approached,  depending  upon  the  care  exer- 
cised in  adjusting  the  position  of  the  lamps  and  the 
distribution,  color,  and  intensity  of  the  light.  Inter- 
esting demonstrations  of  retinal  fatigue  and  after- 
images are  readily  made  with  this  simple  apparatus. 
For  coloring  the  lamps  ordinary  colored  lacquers  are 
satisfactory  if  properly  mixed  to  obtain  the  exact 
hues.  The  aniline  dyes  can  be'  used  with  satis- 
faction. These  colors  are  not  permanent,  but  are 
sufficiently  durable  for  such  an  apparatus.  If  the 
coloring  is  placed  on  separate  plates  of  glass,  it  will 
remain  unfaded  for  a  long  time  with  proper  ventila- 
tion. 

24.  Color  Notation.  —  The  need  for  a  universal 
color  notation  is  admirably  illustrated  by  Munsell 3 
in  quoting  from  a  letter  by  Robert  Louis  Stevenson, 
writing  from  Samoa  to  a  friend  in  London,  as  follows : 

"  Perhaps  in  the  same  way  it  might  amuse  you  to  send  us  any 
pattern  of  wall  paper  that  might  strike  you  as  cheap,  pretty  and  suit- 
able for  a  room  in  a  hot  and  extremely  bright  climate.  It  should 
be  borne  in  mind  that  our  climate  can  be  extremely  dark  too.  Our 
sitting  room  is  to  be  in  varnished  wood.  The  room  I  have  particu- 
larly in  mind  is  a  sort  of  bed  and  sitting  room,  pretty  large,  lit  on 
three  sides,  and  the  colour  in  favour  of  its  proprietor  at  present  is  a 
topazy  yellow.  But  then  with  what  colour  to  relieve  it?  For  a  little 
work-room  of  my  own  at  the  back  I  should  rather  like  to  see  some 
patterns  of  unglossy  —  well  I'll  be  hanged  if  I  can  describe  this  red  - 
it's  not  Turkish  and  it's  not  Roman  and  it's  not  Indian,  but  it  seems 


78  COLOR  AND   ITS  APPLICATIONS 

to  partake  of  the  two  last  and  yet  it  can't  be  either  of  them  because 
it  ought  to  be  able  to  go  with  vermillion.  Ah,  what  a  tangled  web 
we  weave  —  anyway,  with  what  brains  you  have  left,  choose  me  and 

send  me  some  —  many  —  patterns  of  this  exact  shade." 

t. 

Here  is  a  man  accustomed  to  present  his  thoughts 
in  writing  in  a  clear  manner,  yet  he  acknowledges 
failure  in  his  effort  to  describe  colors  and  closes  his 
letter  with  the  request,  perhaps  a  bit  sarcastic,  that 
he  be  sent  "patterns  of  this  exact  shade."  Other 
sciences  have  exact  and  practically  universally  ac- 
cepted terminology.  Music  has  its  well-developed 
notation,  which  is  definite  and  descriptive,  and  quite 
universal  in  adoption,  but  there  is  no  universal 
scheme  of  color  notation.  Colors  are  named  in  very 
inexact,  unwieldy,  and  often  totally  non-descriptive 
terms.  We  have  rose,  Indian  red,  Alice  blue,  pea 
green,  olive  green,  cerise,  taupe,  baby  blue,  Copen- 
hagen blue,  king's  blue,  royal  purple,  invisible  green, 
etc.  Thus  flowers,  vegetables,  cities,  the  savage 
and  the  royal  family,  are  used  to  describe  colors.  Is 
there  a  more  ridiculous  instance  of  neglect?  Those 
who  work  in  color  often  find  themselves  helpless  in 
describing  colors  to  others.  Surely  a  color  notation 
based  upon  color  science  should  be  acceptable,  even 
though  somewhat  empirical.  Musical  notation  is 
somewhat  arbitrary,  yet  it  has  met  with  almost  uni- 
versal adoption.  An  acceptable  color  notation  must 
involve  the  factors  which  influence  the  quality  of  a 
colof,  namely  hue,  saturation,  and  brightness. 

NAn  attempt  was  made  by  Runge  as  early  as  1810 
to  build  up  a  color  notation  by  the  use  of  a  sphere 
with  red,  yellow,  and  blue,  placed  around  the  equator 
and  separated  from  each  other  by  120  degrees,  with 
white  and  black  at  opposite  poles.  Perhaps  the 
greatest  virtue  in  this  attempt  is  the  fact  that  it  was 


COLOR  TERMINOLOGY  79 

one  of  the  early  constructive  efforts.  Chevreul, 
whose  work  on  the  effect  of  simultaneous  contrast  of 
colors  in  the  practical  textile  industry  is  well  known, 
constructed  a  hollow  cylinder  built  up  of  ten  sections 
perpendicular  to  the  axis.  Around  the  upper  section 
red,  yellow,  and  blue  were  equally  spaced.  The 
lowest  cylindrical  section  was  black,  and  the  eight 
intervening  sections  were  graded  from  top  to  bottom 
by  adding  increasing  amounts  of  black.  Munsell 
criticises  these  attempts,  on  account  of  the  yellow 
being  very  light  and  the  blue  being  very  dark,  which 
makes  impossible  any  coherency  in  the  brightness 
scales  of  the  three  colors.  Inasmuch  as  the  bright- 
ness scale  of  the  yellow  in  the  Chevreul  color  cylinder 
increases  much  more  rapidly  from  the  bottom  toward 
the  top  than  the  brightness  scales  for  blue  and  red, 
Munsell  suggests  that  the  yellow  side  of  the  cylinder 
be  increased  in  length.  This  would  result  in  the 
tilting  of  the  sections  more  and  more  as  the  scale 
of  brightness  progressed  from  the  bottom  toward 
the  top.  Perhaps  a  general  criticism  for  most  of  these 
schemes  of  color  notation  is  that  geometrical  figures 
are  chosen  and  the  colors  are  made  to  fit.  The 
latter  method  is  perhaps  partially  justifiable  from 
the  standpoint  of  physical"  measurements.  There 
is  another  viewpoint  in  considering  a  color  notation, 
and  that  is  from  the  standpoint  of  harmony  of  color. 
In  this  treatise  we  are  not  so  much  concerned  with 
the  latter  viewpoint,  but  it  is  of  interest  to  consider 
a  system  of  color  notation  devised  by  Munsell  from 
the  standpoint  of  the  use  of  color  in  painting.  In 
describing  a  color  by  this  system  the  initial  of  the 
name  of  the  color  indicates  the  hue,  and  numerals 
represent  the  saturation  and  brightness.  For  example 
R?  represents  a  color  whose  hue  is  red  and  whose 


80  COLOR  AND  ITS  APPLICATIONS 

saturation  and  brightness  are  respectively  7  and  5. 
The  brightness  scale  is  divided  into  ten  parts,  and 
the  degrees  of  saturation  shown  vary  with  the  bright- 
ness. For  simplicity  ten  hues  are  balanced  around 
the  equator  of  the  sphere  somewhat  after  the  manner 
shown  in  Fig.  22.  The  lower  pole  of  the  sphere  is 
black  and  corresponds  to  zero  on  the  brightness  scale. 
The  upper  pole  is  white  and  corresponds  to  brightness 
10  on  the  same  scale.  On  slicing  off  a  portion  of 
the  sphere  through  a  plane  corresponding  to  a  certain 
brightness,  various  degrees  of  saturation  are  encoun- 
tered. The  saturation  decreases  toward  the  center, 
the  axis  of  the  sphere  consisting  of  a  scale  of  gray 
S.  However,  the  sphere  does  not  completely  satisfy 
Munsell.  He  therefore  constructs  a  *  color  tree'  so 
that  varying  numbers  of  steps  in  saturation  can  be 
represented,  depending  upon  the  hue  and  position 
in  the  brightness  scale.  The  scheme  is  built  up  on 
the  principle  of  the  harmonious  use  of  colors  and  in 
this  respect  departs  somewhat  from  the  scope  of 
this  book,  which  treats  more  with  physical  mixtures, 
regardless  of  the  use  of  colors  in  harmony.  The 
system  is  an  interesting  one  and  is  the  result  of  a 
noteworthy  attempt  to  be  freed  from  a  state  of  color 
anarchy. 

MunselPs  color  tree  is  illustrated  in  simple  form 
in  Fig.  36.  The  base  of  the  tree  is  black,  the  top 
white.  In  the  small  model  illustrated  three  bright- 
ness levels  are  shown,  namely  3,  5,  and  7  in  the 
arbitrary  brightness  (value)  scale.  The  degrees  of 
saturation  shown  vary  with  the  'brightness  level.'  At 
*  level  3J  in  the  brightness  scale  blue  is  shown  to  the 
eighth  degree  of  saturation.  By  the  irregularity  in 
the  contour  of  the  planes  representing  different 
brightness  levels  it  is  seen  that  the  relative  number 


COLOR  TERMINOLOGY 


81 


of  degrees  of  saturation  shown  for  various  colors 
depends  upon  the  brightness  level  under  considera- 
tion. At  brightness  level  3,  PB  (purple-blue)  was 
shown  with  the  highest  degree  of  saturation,  namely, 
8.  At  brightness  level  5,  R  ranked  first  in  degree  of 
saturation,  its  highest  being  10.  At  the  brightness 
level  7,  yellow  was  shown  with  the  highest  degree 

White 


Yellow 


YeL 


Green 


Ye  flow  red 


Red 


Purple-blue 


Red-purple 


Fig.  36.  — The  A.  H.  Munsell  color  tree. 

of  saturation,  namely  8.  For  a  complete  discussion 
of  this  system,  the  reader  is  referred  to  the  original 
description  by  Munsell.3 

Numerous  scales  have  been  devised  involving, 
either  separately  or  combined,  the  factors  hue, 
saturation,  and  brightness.  All  of  these  assist  in 
bringing  order  out  of  chaos,  but  they  constitute  only 
the  first  steps  toward  a  comprehensive  system  of 
color  notation.  The  hues  are  usually  expressed  by 
names  of  spectral  colors  and  purple,  but  the  bright- 
ness is  seldom  more  definite  than  is  found  in  such 
expressions  as  W,  HL,  L,  LL,  M,  HD,  ' D,  LD,  and 


82 


COLOR  AND   ITS  APPLICATIONS 


By  which  represent  white,  high  light,  light,  low  light, 
medium,  high  dark,  dark,  low  dark,  and  black  re- 
spectively. Examples  of  such  charts  (devoid  of  color) 
are  shown  in  Fig.  37.  These  were  taken  from  Book 
VI  of  Prang's  text-book  of  art  education.  Such 


NEUTRAL  VACUC 


CHART -B  CHART  -  C 

Fig.  37.  —  Prang's  color  and  brightness  scales. 

charts  should  pave  the  way  toward  a  final  scientific 
color  notation. 

Another    system    of    color    notation    is    shown    in 
Fig.  38.     This  is  used  by  Ruxton  in  the  mixture  of 

XT  OWS     ~~>A  A.  R.G   O      COLOR^     C  H  A  R.T  I 


Fig.  38.  —  Ruxton's  color  mixture  chart  for  printing  inks. 

printing  inks.  The  chart  is  printed  in  colors,  there 
being  144  colors,  varying  in  hue,  saturation,  and  bright- 
ness. The  terminology  is  somewhat  different  than 
used  in  this  text.  The  144  colors  are  obtained  from 
six  fundamental  colors,  namely  red,  orange,  yellow, 
green,  blue,  and  purple.  These  six  colors  are  de- 
scribed as  spectral  colors,  so  it  is  likely  that  purple 


COLOR  TERMINOLOGY  83 

is  the  name  applied  to  a  color  meant  to  be  violet. 
The  starting  point  in  obtaining  the  total  array  of 
colors  is  in  the  bottom  row  of  Section  3.  The  larger 
rectangles  represent  the  six  fundamental  colors,  which 
are  the  purest  or  most  saturated  on  the  chart.  The 
fundamental  red  is  marked  820.  The  small  square 
areas  represent  the  intermediate  hues  and  are  ob- 
tained by  mixing  the  fundamentals  on  either  side. 
Red-orange  for  instance  is  obtained  by  mixing  red 
and  orange  (820  and  840).  The  three  horizontal 
rows  above  this  row  of  twelve  colors  are  made  by 
adding  white  to  the  colors  of  the  bottom  row.  Thus 
in  the  top  row  are  found  the  least  saturated  colors 
in  Section  3.  Two  degrees  of  saturation  lie  between 
the  top  and  bottom  rows.  Thus  in  Section  3  there 
are  48  colors,  six  fundamental  colors  increased  to 
twelve  by  mixing  adjacent  fundamentals  (red  and 
violet  are  mixed,  producing  810)  and  these  twelve 
colors  decreased  in  saturation  in  three  steps  by  the 
addition  of  white.  Sections  1  and  2  are  produced 
by  adding  black  to  the  corresponding  colors  in  Section 
3,  thus  reducing  their  brightness.  In  Section  1  are 
the  colors  of  lowest  brightness.  These  are  named 
'hues'  but  in  a  different  sense  than  that  in  which 
the  term  'hue'  is  employed  here.  They  could  be 
termed  'Values'  with  better  consistency.  The  'bi- 
hues'  (bi-values)  in  Section  2  are  obtained  by 
mixing  one  part  by  weight  of  the  colors  in  Section  3 
to  one  part  of  the  corresponding  color  in  Section  1. 
Thus  it  is  seen  that  in  a  more  correct  sense  there 
are  twelve  hues  represented  on  the  chart  (bottom 
row  in  Section  3).  With  the  hue  and  brightness 
constant  saturation  is  found  to  be  present  in  four 
degrees  (moving  vertically  in  Section  3).  With  the 
hue  and  saturation  constant,  the  brightness  is  found 


84  COLOR  AND  ITS  APPLICATIONS 

to  be  present  in  three  values  (moving  from  left  to 
right,  Sections  3,  2,  1,  so-called  *  colors,'  'bi-hues' 
and  'hues').  Besides  these  there  are  72  other  colors 
in  which  brightness  and  saturation  appear  in  six 
combinations  for  each  of  the  12  hues.  In  other 
words,  in  a  broad  sense  there  are  present  144  colors 
made  up  of  twelve  hues  by  varying  the  brightness 
and  saturation.  Six  of  the  twelve  hues  are  made  by 
mixture  of  the  adjacent  hues  in  the  bottom  row  of 
large  rectangles  in  Section  3.  Each  rectangle  being 
numbered,  the  chart  systematizes  the  mixture  of 
printing  inks.  Such  progress  is  commendable  and 
highly  desirable,  even  though  empirical. 

There  are  many  other  methods,  but  these  few 
have  been  cited  to  show  the  lack  of  standardization 
of  color  notation  and  to  illustrate  that  a  system 
however  empirical  is  just  as  desirable  for  the  de- 
scription of  color  as  a  system  is  for  music  notation. 
There  is  much  yet  to  be  done  before  a  system  of  color 
notation  is  devised  which  will  be  universally  adopted. 
First  there  should  be  some  definite  terms  adopted 
descriptive  of  the  factors  influencing  the  quantity 
of  a  color,  namely '  hue,' '  saturation,' '  brightness.'  The 
term  'hue'  is  used  in  a  more  definite  sense  than  the 
terms  applied  to  the  two  other  factors.  For  satura- 
tion the  terms  'chroma,'  'purity,'  'intensity,'  and  others 
are  being  used.  For  brightness  the  terms  'luminos- 
ity,' 'value,'  'hues'  or  'bi-hues,'  and  others  are 
being  used.  Purples  are  often  called  violets  or  reds. 
These  are  examples  of  usage  from  which  general 
confusion  arises.  The  problem  of  color  terminology 
does  not  defy  solution.  As  a  matter  of  fact  all  the 
quantities  involved  in  a  scientific  system  of  notation 
are  readily  measurable.  Hue,  saturation,  and  bright- 
ness are  easily  determined.  The  available  hues, 


COLOR  TERMINOLOGY  85 

with  the  exception  of  purple,  are  invariable,  consisting 
of  the  spectral  hues.  Scales  of  brightness  (value) 
can  be  divided  into  any  given  number  of  parts  and 
named  in  some  consistent  manner.  The  use  of  the 
terms  'high  light,'  'low  light,'  'medium,'  'low  dark,' 
etc.,  is  perhaps  satisfactory,  but  the  brightnesses  that 
they  represent  should  be  standardized  in  absolute 
measurements  in  order  to  produce  a  universal  scale 
of  relative  brightnesses.  In  fact  all  the  terms  re- 
quired in  a  satisfactory  and  scientific  system  of  color 
notation  can  be  measured  for  their  absolute  values. 
This  would  reduce  the  systems  to  one  basis.  Such 
a  universal  system  must  certainly  be  adopted  even- 
tually, and  those  interested  in  color  should  put  forth 
effort  to  hasten  the  day. 

REFERENCES 

1.  Primer  of  Psychology,  1899,  p.  41. 

2.  Psych.  Rev.  20,  May,  1913. 

3.  A  Color  Notation. 

OTHER    REFERENCES 

Sir  William  Abney,  Color  Mixture  and  Measurement. 
O.  N.  Rood,  Textbook  on  Color. 

J.   G.   Hagen,  Various   Scales  for  Color-Estimates,  Astrophys. 
Jour.  1911,  34,  p.  261. 

K.  Zindler,  Color  Pyramid,  Zeit.  f.  Psych.  1899,  20,  p.  225. 
R.  Ridgeway,  Color  Scales. 


CHAPTER   V 
ANALYSIS   OF  COLOR 

25.  The  Spectroscope.  -  -  As  already  indicated, 
colors  can  be  analyzed  in  various  ways.  The  method 
adopted  in  a  given  case  will  naturally  depend  upon 
data  desired.  The  spectroscope  affords  a  simple 
means  of  examining  colored  light,  but  the  results  of 
the  visual  inspection  are  only  qualitative.  There 
are  various  designs  of  spectroscopes  available,  all 
based  upon  either  the  principle  of  the  prism  or  of 
the  diffraction  grating  (#8  and  9).  An  ordinary  prism 
spectroscope  can  be  converted  into  a  direct-vision 
instrument  by  combining  two  prisms  made  from 
different  kinds  of  glass,  so  that  dispersion  is  obtained 
for  a  certain  ray  without  deviation.  Crown  and 
flint  glasses  differ  in  refractive  index  (Fig.  8),  hence 
if  prisms  of  each  of  these  two  glasses  be  made  of 
proper  refractive  angles  and  combined  so  that  their 

separate   deviations   prac- 


AF/\            |L]  tically   annul   each   other, 

'      v     ^ LL I    say,  for  the   sodium  line, 

Fig.  39.  — A  direct-vision  prism  spec-     dispersion     is     produced 

troscope-  without  deviation  for  this 
— -. ray.     Such  a  simple  spec- 
troscope is  shown  in  Fig. 
1    39.     A   simple    diffraction 

Fig.  40.  —  A  simple  grating  spectro-      grating      SpCCtrOSCOpe     Can 

be  readily  made,  as  shown 

in  Fig.  40.     A  replica  of  a  diffraction  grating  (#9)  is 
placed  between  two  pieces  of  plate  glass  at  G.     By 


ANALYSIS  OF  COLOR  87 

placing  a  lens  at  L  the  instrument  is  considerably 
shortened,  so  that  it  can  readily  be  made  of  a  pocket 
size.  These,  the  simplest  forms  of  spectroscopes, 
are  only  useful  for  rough  qualitative  analysis  of  the 
spectral  character  of  light  emitted  by  light  sources 
or  transmitted  or  reflected  by  colored  media.  A 
convenient  form  of  spectroscope  for  qualitative  analy- 
sis is  the  comparison  spectroscope.  This  contains 
two  or  three  distinct  optical  systems,  so  that  two  or 
three  spectra  may  be  viewed  in  juxtaposition.  Such 
an  instrument  might  be  considered  as  roughly  quan- 
titative in  its  analyses,  owing  to  the  opportunity  of 
estimating  relative  intensities  of  a  given  light  ray 
in  the  two  or  three  spectra. 

More  elaborate  spectrometers  will  not  be  con- 
sidered here,  for  the  function  of  the  spectrometer 
is  for  qualitative  analysis.  However,  in  this  respect 
such  instruments  are  of  considerable  value  in  color 
work.  Photographic  accessories  are  readily  attached 
in  place  of  the  eyepiece.  If  an  absorption  wedge  be 
placed  before  the  slit,  so  that  its  transmission  varies 
along  the  length  of  the  slit,  the  spectrograms  will 
roughly  indicate  the  relative  spectral  distribution  of 
energy,  providing  proper  filters  are  used  to  allow  for 
the  variation  in  plate  sensibility.  Sometimes  it  is 
advantageous  to  compensate  for  the  unequal  spectral 
distribution  of  energy  in  the  illuminant,  especially 
in  the  examination  of  colored  media. 

26.  The  Spectrophotometer. — This  instrument 
consists  in  principle  of  two  spectroscopes,  arranged 
so  that  the  intensity  of  rays  of  the  same  wave-length 
in  the  two  spectra  can  be  photometrically  compared. 
The  results  obtained  are  quantitative.  A  diagram- 
matic sketch  of  the  optical  system  of  a  Spectro- 
photometer is  shown  in  Fig.  41.  Light  enters  the 


88 


COLOR  AND  ITS  APPLICATIONS 


instrument  from  two  sources  at  the  slits  S  and  S', 
respectively.  At  L  is  a  Lummer-Brodhun  photometer 
cube  so  constructed  that  through  a 
part  of  the  field  light  rays  are  trans- 
mitted directly  from  S'  to  the  prism 
P  and  from  the  remainder  of  the 
field  light  rays  from  S  are  reflected 
s  toward  the  prism.  After  being  dis- 
persed by  the  prism  the  colored  rays 
pass  on  to  the  eye  placed  at  T.  The 
wave-length  of  these  rays  depends 
upon  the  angular  position  of  the  prism 
which  can  be  rotated.  The  photom- 
eter field  is  similar  to  that  viewed 
in  an  ordinary  Lummer-Brodhun 
photometer. 

A  small  direct-vision  comparison  spectroscope  is 
of  considerable  use  in  color  work.  Such  an  instru- 
ment designed  by  Nutting  l  contains  a  pair  of  Nicol 
prisms,  NN,  for  altering  the  brightness  of  one  of 
the  spectra,  as  shown  in  Fig.  42.  A  right-angled 


Fig.  41.  —  The  spectro- 
photometer. 


Fig.  42.  —  The  Nutting  pocket  spectrophotometer. 

prism,  /?,  reflects  light  into  the  slit  from  one  of  the 
sources.  The  instrument,  which  is  called  a  pocket 
spectrophotometer,  in  itself  is  merely  for  qualitative 
analysis.  It  can  be  set  up  permanently  and  used 
for  quantitative  measurements.  However,  in  order 
to  make  such  an  instrument  portable  and  compact, 
yet  available  for  obtaining  qualitative  data,  the  author 
devised  the  attachments  shown  in  Fig.  43.  An 
attachment,  A,  containing  a  miniature  tungsten  lamp 


ANALYSIS   OF   COLOR  89 

which  illuminates  a  ground  flashed-opal  glass,  can 
be  removed  from  its  present  position  if  desired  and 
attached  at  B.  When  the  comparison  source  in  A 


Fig.  43.  —  A  small  portable  spectrophotometer  for  quantitative  analysis. 

is  in  the  position  shown  the  unknown  source  is  placed 
at  B.  The  sleeve,  O,  was  placed  on  the  instrument 
to  support  A.  S  controls  the  slit  widths.  A  mi- 
crometer screw  with  a  graduated  scale  and  drum  is 
attached  at  M  to  the  slide  containing  the  observing 
slit,  C,  which  is  moved  across  the  image  of  the 
spectrum.  The  drum  is  calibrated  in  terms  of  wave- 
lengths and  the  scale  of  the  revolving  Nicol  prism 
in  terms  of  transmission  of  light.  The  current 
through  the  lamp  is  obtained  from  a  battery  controlled 
by  a  rheostat  and  is  measured  by  means  of  a  small 
ammeter.  The  range  of  the  instrument  can  be 
extended  by  varying  the  current  through  the  lamp. 
This  photometric  field  is  not  as  satisfactory  as  might 
be  desired,  for  it  consists  of  two  narrow  bands  juxta- 
posed at  their  ends.  The  instrument  is  very  small, 


90 


COLOR  AND   ITS  APPLICATIONS 


less  than  8  inches  long,  is  mounted  on  a  tripod,  and 
is  really  portable. 

There  are  many  designs  of  spectrophotometers, 
but  all  have  the  same  object.  It  is  necessary  to 
be  able  to  vary  the  luminous  intensity  of  one-half 
of  the  photometer  field.  This  is  done  by  varying  the 
position  of  one  of  the  light  sources,  by  the  use  of 
Nicol  prisms,  by  a  neutral  tint  absorbing  wedge,  or 

by  sectored  disks.  A 
convenient  means  is  the 
variable  sectored  disk 
developed  by  Hyde,2  a 
diagram  of  which  is 
shown  in  Fig.  44.  The 
disk  is  mounted  upon  a 
motor-driven  shaft  and 
arranged  to  be  moved 
horizontally  in  its  plane 
along  the  line  CD  in 
front  of  the  slit  S,  by 
means  of  a  micrometer 
screw.  The  transmission  is  nearly  proportional  to 
the  lateral  displacement. 

By  means  of  the  spectrophotometer,  results  can  be 
obtained  directly  in  terms  of  relative  energy  such  as 
are  plotted  in  Fig.  5  (Table  II)  and  Figs.  122,  123. 
In  this  case  the  various  rays  in  the  unknown  spectrum 
are  compared  directly  with  the  corresponding  rays  in 
a  spectrum  of  known  distribution  of  energy.  As  has 
been  previously  stated  the  spectrophotometer  is  an 
analytical  instrument,  and  by  its  use  the  spectral 
character  of  the  light  reflected  or  transmitted  by 
colored  media  is  readily  obtained.  An  example  of  its 
use  and  a  practical  means  of  greatly  reducing  the 
number  of  readings  is  given  below.  During  the 


Fig.  44.  — The  variable   sectored   disk. 
(After  Hyde.) 


ANALYSIS   OF   COLOR 


91 


development  of  a  glass  which  could  be  used  with  the 
tungsten  lamp  to  produce  artificial  daylight 3  the 
procedure  involved  the  examination  of  many  glasses 
containing  various  proportions  of  coloring  ingredients. 
A  glass  which  proved  unsatisfactory  at  the  thickness 
at  hand  might  be  found  satisfactory  at  another  thick- 
ness. Therefore  it  was  necessary  to  grind  and  polish 
the  samples  as  they  came  from  the  glass  factory  into 
many  different  thicknesses  or  in  the  form  of  a  wedge. 


§    0.01 


01234567 
THICKNESS  OF  GLASS  IN  MILLIMETERS 


Fig.  45.  —  Scheme  for  reducing  the  amount  of  spectrophotometric  work  in  exam- 
ining transparent  colored  media. 

This  necessitated  making  a  set  of  spectrophotometric 
readings  for  a  considerable  range  of  thicknesses.  By 
utilizing  the  law  relating  transmission  and  thickness 
(density  of  coloring  matter)  of  the  glass,  namely  7  = 
70e~e'z,  a  simple  method  was  devised.  70  represents 
the  original  intensity  of  light  of  a  certain  wave-length 
and  7,  its  intensity  after  traversing  a  thickness  d  of 
the  colored  glass  and  e,  the  extinction  coefficient. 
By  considering  the  reflection  from  the  two  surfaces 
of  the  glass  a  relation  was  deduced  in  the  form  of 


92  COLOR  AND   ITS  APPLICATIONS 

Log  T  =  log  0.92  +kd  (where  T  is  the  transmission,  d 
the  thickness  and  k  a  constant)  which  is  sufficiently 
accurate  for  ordinary  purposes  in  the  spectrophoto- 
metric  analysis  of  the  transmission  characteristics 
of  colored  glasses  and  other  media.  The  term  'log 
0.92'  can  be  eliminated  by  obtaining  the  transmis- 
sion of  the  colored  glass  in  terms  of  a  clear  glass 
if  so  desired.  This  method  necessitates  an  analysis 
of  only  one  thickness,  for,  on  plotting  these  data  on 
logarithmic  paper,  as  shown  in  Fig.  45,  the  data  for 
various  other  thicknesses  (even  thicker  than  the 
sample)  are  readily  obtained.  Proof  of  the  accuracy 
of  this  method  is  shown  by  the  fact  that  the  circles 
which  represent  data  obtained  on  the  same  sample 
of  glass  at  five  different  thicknesses  lie  close  to  the 
straight  lines  indicated  by  the  mathematical  relation 
expressed  above.  See  Chapter  XVII. 

The  spectrophotometric  examination  of  colored 
media  is  valuable  inasmuch  as  the  eye  not  being 
analytic,  other  methods  fail  to  reveal  the  true  spectral 
character  of  the  light  emitted  by  the  colored  medium. 
This  was  demonstrated  by  the  three  yellows  in  Fig. 
17  which  appeared  of  the  same  hue  (and  practically 
of  the  same  saturation),  but  differed  greatly  in  spectral 
character. 

A  spectrophotometer  is  an  elaborate  and  expensive 
instrument,  therefore  where  the  need  for  such  an 
instrument  is  not  great  enough  to  warrant  its  pur- 
chase, an  ordinary  spectrometer  with  modifications 
can  be  made  to  serve  the  purpose.  There  are  various 
ways  of  converting  ordinary  spectrometers  into  in- 
struments satisfactory  for  spectrophotometric  work. 
A  double  bilateral  slit  and  a  combination  prism  for 
transmitting  and  reflecting  respectively  two  juxta- 
posed beams  of  light  from  the  different  sources  into 


ANALYSIS  OF  COLOR 


93 


1  S 


the  collimator  is  a  ready  means  of  converting  a 
spectrometer  into  a  spectrophotometer.  However,  the 
comparison  field  which  consists  of  narrow  lines  juxta- 
posed endwise  is  not  very 
satisfactory.  Abney  used 
the  scheme  illustrated  in 
Fig.  46  in  his  early  studies 
in  color.  Two  slits,  SS, 
were  placed  in  a  plane  at 
right  angles  to  the  col- 
limator. One  slit  was  be- 


Fig.  46.  —  Abney's  spectrophotometric 
attachment  for  a  spectrometer. 


low  the  other,  so  that  their  respective  images  could  be 
reflected  toward  the  collimating  lens  by  the  two  right- 
angled  prisms  which  were  placed  one  below  the  other. 
This  arrangement  no  doubt  yielded  a  photometric 
field  which  was  not  divided  by  an  invisible  line,  as  is 
desirable  for  high  sensibility.  #6 


cr    -b 


-3- 


Fig.  47. — Ives'  spectrophotometric  attachment  for  a  spectrometer. 

A  more  satisfactory  method  is  illustrated  in  Fig. 
47.  This  arrangement,  used  by  Ives,4  was  designed 
chiefly  to  avoid  the  errors  due  to  instruments  having 
two  collimators  becoming  asymmetrical  and  also  to 
avoid  errors  due  to  scattered  light.  At  1  in  Fig. 


94  COLOR  AND  ITS  APPLICATIONS 

47  is  placed  a  combination  of  two  right-angle  prisms 
cemented  together.  The  face  b  is  entirely  silvered 
and  face  a  silvered  halfway  up.  A  lens  at  2  forms 
an  image  in  the  field  of  the  telescope  tube  at  3  which 
is  observed  by  means  of  an  ocular  lens  at  4.  The  two 
light  sources  are  placed  at  6  and  7  respectively.  A 
large  monochromatic  field  is  obtained  which  is  equally 
affected  by  scattered  light  if  any  is  present.  Further- 
more colored  glasses  can  be  judiciously  used  to  elimi^ 
nate  scattered  light  if  necessary. 

A  further  improvement  of  the  foregoing  attach- 
ment, which  was  added  by  Nutting,5  is  illustrated  in 
Fig.  48.  The  attachment  consists  of  two  reflecting 

prisms,  Pi  and  P2,  two 
Nicol  prisms,  Ni  and  N2y 
and  a  lens  arranged  as 
\  _  \/  |  shown  in  the  figure.  The 
-£^/p*  '  whole  can  be  attached  to 
the  slit  of  any  spectrometer. 
The  essential  factor  is  that 
a  real  image  of  the  photo- 
metric field  (the  common 
surface  of  the  two  reflect- 

the  slit  by  an  achromatic 
lens  and  is  thus  brought  into  the  plane  of  the  slit. 
The  two  beams  of  light  to  be  compared,  one  passing 
through  a  portion  of  the  photometric  surface  and  the 
other  being  reflected  by  the  other  portion  which  is 
silvered,  are  brought  to  a  brightness  balance  for 
any  wave-length  by  rotating  the  Nicol  prism  M. 
High  sensibility  is  claimed  by  Nutting  for  an  instru- 
ment of  this  type. 

27.  The  Monochromatic  Colorimeter.  —  Colorim- 
eters vary  in  design,  depending  upon  the  data  to  be 


ANALYSIS  OF  COLOR  95 

obtained.  In  some  industrial  processes  tintometers 
are  employed  which  determine  the  color  of  substances 
in  terms  of  arbitrary  standards.  Such  instruments  are 
colorimeters,  but  give  no  quantitative  analyses  of  the 
colors.  Their  purpose  is  largely  to  keep  the  product 
within  certain  limits  as  to  color,  but  they  perhaps  serve 
the  purpose  in  many  of  these  cases  as  satisfactorily  as 
a  more  complex  instrument.  Of  the  instruments  that 
analyze  colors  into  the  three  terms  'hue,'  'satura- 
tion' and  'brightness,'  the  Nutting6  colorimeter,  being 
of  the  latest  type,  has  been  chosen  for  description.  The 
optical  system  of  this  instrument,  which  has  been 
called  a  monochromatic  colorimeter,  is  shown  in  Fig. 
49.  Light  entering  the  slit  of  collimator,  A,  which 
is  movable,  traverses  the 
prism  and  is  dispersed  by 
prism  P  into  its  spectral 
components,  thus  furnish- 
ing  the  measurement  of 
hue  of  the  unknown  light 
which  enters  through  the 
slit  of  collimator  C  and  is 
reflected  by  a  portion  of 
the  diagonal  surface  in  L, 

which  is  a  Lummer-Brodhun  photometer  cube.  White 
light  enters  the  slit  of  collimator  B  and  is  reflected  by 
the  prism  face  and  joins  a  portion  of  the  beam  from 
A.  The  eye  placed  at  the  ocular  slit  in  D  sees  an 
ordinary  photometric  field,  the  two  parts  of  which 
can  be  matched  in  hue,  saturation,  and  brightness. 
The  hue  is  matched  by  varying  the  angular  position" 
of  A  and  the  saturation  by  varying  the  amount  of 
white  light  added.  The  amounts  of  light  entering 
the  slits  can  be  varied  by  changing  the  slit  widths, 
by  rotating  sectors,  or  by  rotating  one  of  a  pair  of 


96 


COLOR  AND   ITS  APPLICATIONS 


Nicol  prisms  placed  just  inside  the  slits.  In  analyz- 
ing a  purple,  for  which  no  spectral  match  in  hue 
exists,  a  spectral  color  is  mixed  with  the  unknown, 
the  remaining  procedure  being  obvious.  The  later 
instruments  have  been  altered  somewhat  in  con- 
struction, but  the  principle  remains  the  same.  The 
accuracy  with  which  the  dominant  hue  is  obtainable 
is  claimed  to  be  about  .001  to  .002/*  except  in  the 
extreme  regions  of  the  spectrum,  for  very  unsaturated 
colors  and  dark  shades.  Data  obtained  by  Nutting  6 
are  given  in  Table  V. 


TABLE  V 


Materials 

Hue 

Per  cent 
white 

Reflection 
coefficient 

Sulphur 

0.571/z 

48 

0.80 

Cork            

.586 

56 

.26 

Dandelion              

.580 

9 

Tobacco  leaf  (medium) 

.597 

65 

.14 

Chocolate 

.595 

70 

05 

Butter,  light       

.580 

45 

Butter,  dark  •           

.580 

28 

.64 

Navy  blue  (U.  S.) 

.472 

90 

.019 

Paris  green 

.511 

56 

386 

Manila  paper       

.682 

65 

.57 

CoDDer 

.597 

70 

.23 

Brass  light 

.575 

60 

.32 

Brass  dark 

.583 

61 

25 

Gold,  medium       

.591 

64 

.21 

Data  obtained  by  Abney  7  in  the  analysis  of  the 
color  of  glasses  and  pigments  are  presented  in  Table 
VI. 

In  Table  VII  are  given  some  data  on  the  color  of 
illuminants  obtained  by  L.  A.  Jones  8  with  the  mono- 
chromatic colorimeter. 


ANALYSIS   OF   COLOR 


97 


TABLE   VI 


Hue 

Saturation 

Brightness 

Glasses 

Dominant 
hue 

Per  cent 
white 

Transmis- 
sion coef- 
ficient 

Ruby                                                            .    -  . 

0.622,u 

2 

0  131 

Canary 

.585 

26 

.820 

Bottle-green                                              

.551 

31 

106 

Signal-green 

.4925 

32 

069 

n         « 

.510 

61 

194 

Cobalt                                                      

.4675 

42 

.038 

Pigments 

Dominant 
hue 

Per  cent 
white 

Reflection 
coefficient 

Vermillion  

0.610M 

2.5 

0.148 

Emerald-green  

.522 

59 

.227 

French  ultramarine  blue 

.472 

61 

.044 

Brown  paper  '. 

.594 

50 

.25 

Orange           

.5915 

4 

.625 

Chrome-yellow 

.5835 

26 

.777 

Blue-green 

.5005 

42.5 

.148 

Eosine  dye  

.640 

72 

.447 

Cobalt-blue 

.482 

55.5 

.145 

TABLE   VII 


Source 

Per  cent 
white 

Hue 

Sunlight                              

100 

Average  clear  sky 

60 

0472u 

Standard  candle 

13 

593 

Hefner  lamp   

14 

.593 

Pentane  lamp           .                     ... 

15 

.592 

Tungsten  glow  lamp,  1.25  w.  p.  c. 

35 

.588 

Carbon  glow  lamp,  3.8  w.  p.  c.  

25 

.5915 

Nernst  glower,  1.5  w.  p.  c  

31 

.5867 

Nitrogen-filled  tungsten  lamp,  1.00  w.  p.  m.  h.  c  
Nitrogen-filled  tungsten  lamp,  0.5  w.  p.  m.  h.  c  
Nitrogen-filled  tungsten  lamp,  0.35  w.  p.  m.  h.  c  
Mercury  vapor  arc  

34 
45 
53 
70 

.586 
.5845 
.584 
.490 

Helium  tube  ....                   .    . 

32 

.598 

Neon  tube 

6 

.605 

Crater  of  carbon  arc  at  1.8  amperes  .... 
Crater  of  carbon  arc  at  3.2  amperes  

59 
62 

.5846 
.5846 

Crater  of  carbon  arc  at  5.0  amperes  

67 

5834 

Acetylene  flame  (flat) 

36 

5855 

98  COLOR  AND   ITS  APPLICATIONS 

In  colorimetric  work  a  standard  white  light  is 
necessary.  Jones  used  noon  sunlight,  which  he  found 
to  be  constant  in  color  from  9  A.M.  to  3  P.M.,  the  obser- 
vations extending  over  several  weeks.  This  light  was 
reflected  into  the  instrument  from  a  magnesium  car- 
bonate block. 

Many  interesting  studies  in  color-mixture  can 
be  made  with  such  a  colorimeter.  An  example  is 
found  in  the  work  of  L.  A.  Jones  9  in  the  analysis  of 
mixtures  of  two  component  colors.  Filters  were 
chosen  in  several  cases  practically  complementary 
in  color.  These  filters  were  in  the  form  of  sectors 
of  a  circle  and  of  equal  angular  extent.  An  opaque 
sector  equal  in  size  to  one  of  the  filters  was  varied 
in  position  over  the  sectors,  so  that  they  could  be  left 
open  in  any  desired  proportions.  Lights  passing 
through  these  filters  were  mixed  in  a  complete  range 
of  ratios  and  the  resultant  mixtures  were  examined 
by  means  of  a  monochromatic  colorimeter  for  hue 
and  saturation  or  per  cent  white.  For  example,  we 
will  choose  one  of  the  pairs  of  filters,  a  red  and  blue- 
green  of  dominant  hues  0.624^  and  0.497/x  respec- 
tively. The  saturation  or  purity  of  the  colors  are 
indicated  by  the  per  cent  of  white  light  (noon  sun- 
light) that  each  transmitted,  these  being  for  the  red 
and  blue-green  filters  respectively  3.3%  and  28%. 
The  transmission  coefficients  of  the  two  filters  were 
respectively  24%  and  16%.  The  data  obtained  in 
analyzing  various  mixtures  of  the  two  colored  lights 
are  shown  in  Fig.  50.  It  is  seen  that  practically  only 
two  hues  are  obtained  in  a  complete  range  of  mix- 
tures and  these  are  the  dominant  hues  of  the  re- 
spective colored  lights.  The  dominant  hue  of  the 
mixtures  changes  abruptly  from  the  hue  of  one  of 
the  colored  lights  to  that  of  the  other  at  the  point 


ANALYSIS   OF  COLOR 


99 


near  where  the  mixture  contains  the  maximum 
amount  of  white  light,  or  in  other  words  where  the 
two  lights  are  nearest  to  being  complementary.  The 
per  cent  white  reaches  a  maximum  of  95%  (indi- 
cating that  the  colored  lights  are  here  practically  com- 
plementary) when  the  blue-green  filter  was  open 
about  62%  and  the  red  filter  about  38%.  A  con- 
clusion, among  others  drawn  by  Jones  from  this  in- 


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COM  POSITION  OF  MIXTURE  IN  PER  CENT.  OF  BLUE-GREEN  FILTER 

Fig.  50.  —  Analysis  of  two-component  color-mixtures. 

vestigation,  is  that  it  is  not  possible  with  two  filters 
that  are  complementary  or  nearly  so  to  produce  mix- 
tures that  show  appreciable  color  of  more  than  two 
dominant  hues,  these  hues  being  the  dominant  hues 
of  the  two  components  of  the  mixture.  He  points 
out  other  possibilities  for  this  colorimeter  in  investiga- 
tions in  color-mixture. 

The  author  has  used  an  arrangement  diagram- 
matically  shown  in  Fig.  51  for  the  study  of  various 
problems,  chiefly  that  of  the  influence  of  saturation 
of  color  in  heterochromatic  photometry.  This  ar- 
rangement has  all  the  essentials  of  a  colorimeter  for 


100  COLOR  AND   ITS  APPLICATIONS 

analyzing  colors  into  hue,  saturation,  and  brightness. 
Light  from  a  source  L  emitting  light  of  a  continuous 
spectrum  enters  the  collimator  of  a  Hilger  spectro- 
scope and  is  dispersed  by  the  prism.  A  standard 
white  light  illuminates  the  non-selective  ground  opal 
glass,  O,  an  image  of  which  is  reflected  into  the 
objective  telescope  from  the  prism  face  as  shown. 
A  white  sectored  disk,  Z>,  which  is  smoked  with 
magnesium  oxide  by  holding  near  a  burning  mag- 
nesium ribbon,  is  placed  so  that  it  bisects  the  field, 


w 


u 

Fig.  51.  —  A  simple  method  of  converting  a  spectrometer  into  a  combined  mono- 
chromatic colorimeter,  direct  comparison  photometer,  flicker  photometer, 
and  spectrophotometer. 

F,  vertically.  If  the  edges  of  the  sectors  are  beveled 
and  well  sharpened,  the  dividing  line  can  be  made  to 
disappear  almost  completely.  The  light  from  the 
unknown,  £7,  is  reflected  from  the  disk.  By  varying 
the  intensity  of  the  various  lights  the  desired  meas- 
urements can  be  made.  The  hue  is  determined  by 
the  position  of  the  wave-length  drum;  the  amount  of 
white  light  can  be  measured  by  comparing  with  a 
standard  at  U  or  by  a  previous  calibration.  The 
brightness  can  be  measured  either  by  the  direct 
comparison  or  the  flicker  method  of  photometry  (#  55). 
The  sectored  disk  provided  with  a  motor  drive  is  in 
reality  a  Whitman-disk  flicker  photometer.  As  al- 
ready stated  the  arrangement  was  originally  devised 
for  another  investigation;  however,  it  readily  serves 


ANALYSIS   OF  COLOR  101 

the  requirements  of  a  monochromatic  colorimeter. 
Transparent  colored  media  can  be  illuminated  by  a 
standard  white  light  placed  at  U.  Likewise  opaque 
colored  media  can  be  placed  on  the  disk  D  and 
illuminated  by  a  white  light. 

28.  The  Tri-color  Method.  --It  is  well  known 
that  any  color  can  be  matched  in  hue  by  mixing  the 
three  primary  colors,  red,  green,  and  blue,  in  proper 
proportions.  The  Young-Helmholtz  theory  of  color 
vision  is  largely  based  on  this  experimental  fact  (#47). 
Kdenig  found,  by  a  rather  complex  method,  the  rela- 
tive amounts  of  the  three  primary  color  sensations 
aroused  by  the  various  spectral  colors  and  determined 

iA 


Fig.  52.  —  Illustrating  the  principle  of  the  Maxwell  *  color  box.* 

the  so-called  sensation  curves  of  the  eye  (Fig.  54). 
Maxwell  was  one  of  the  first  to  obtain  quantitative 
data  in  matching  colors  by  a  mixture  of  three  primary 
spectral  colors.  His  apparatus,  known  as  the  *  color 
box,'  though  somewhat  more  complex,  involved  the 
fundamental  principle  shown  in  Fig.  52,  and  is  based 
upon  the  fact  that  an  optical  path  is  'reversible.' 
For  example  if  a  spectrum  is  formed  at  A  by  means 
of  a  collimator  and  a  prism  by  light  entering  the  slit, 
S,  of  the  collimator  and  traversing  a  prism,  we  can 
obtain  a  patch  of  light  of  any  color  by  placing  slits, 
/?,  G,  and  5,  in  the  spectrum  and  combining  the 
light  from  these  on  a  distant  screen.  Conversely, 
if  the  latter  slits  be  illuminated  with  white  light  on 
looking  into  the  collimator  slit,  S,  the  prism  face  will 
appear  of  a  color  which  is  the  result  of  the  mixture 
of  the  colors  of  the  slits  which,  in  the  first  case,  were 


102  'COLOR  AND   ITS  APPLICATIONS 

combined  by  means  of  a  lens  into  one  colored  patch 
on  the  distant  screen.  Hence,  instead  of  forming  a 
spectrum  at  A,  and  producing  colored  light  by  mix- 
tures of  /?,  G,  and  B,  by  slits  placed  at  these  points 
and  combining  the  three  colored  lights,  Maxwell 
adopted  the  reverse  process.  He  illuminated  the 
three  slits  by  sunlight  reflected  from  a  white  diffus- 
ing surface  placed  in  front  of  them,  and  on  looking 
into  the  slit,  S,  he  saw  the  prismfface  appear  in  colors 
corresponding  to  the  positions  and  proportions  of  R, 
G,  and  B.  This  composite  color  he  compared  with 
the  original  white  light  or  any  colored  light.  The 
Maxwell  color  box  was  actually  constructed  in  a 
different  manner,  but  the  principles  involved  are  the 
same  as  indicated  above.  By  means  of  this  instru- 
ment he  obtained  many  color  equations  of  the  form 
xR  +  yG  +  zB  =  C.  By  a  similar  method  Koenig 
obtained  data  which  resulted  in  the  production  of  the 
so-called  sensation  curves  of  the  eye.  Abney  has 
employed  the  method  in  a  great  deal  of  work  in  color 
analysis,  including  the  study  of  color  vision,  the  analy- 
sis of  pigment  colors,  and  of  the  color  of  illuminants. 
^A  colorimeter  based  upon  the  tri-color  method 
of  analysis  was  developed  by  F.  E.  Ives.10  Instead 
of  spectral  colors,  red,  green,  and  blue  colored  filters 
are  employed  in  this  instrument,  which  is  illustrated 
in  Fig.  53.  By  means  of  this  instrument  colors  are 
analyzed  in  terms  of  the  colors  of  the  filters  /?,  G, 
and  B.  These  can  be  reduced  to  sensation  values 
as  shown  later.  D  is  a  variable  slit  which  is  illumi- 
nated by  light  of  the  color  to  be  analyzed  and  A  is 
an  optical  mixing  wheel  consisting  of  twelve  convex 
lenses  arranged  to  rotate.  By  means  of  this  wheel 
the  various  amounts  of  the  red,  green,  and  blue  com- 
ponents are  mixed  to  match  the  light  from  D.  F 


ANALYSIS   OF  COLOR 


103 


is  the  field  lens  and  C  a  prism  or  small  angle  which 
divides  the  photometric  field  by  a  sharp  line  in  the 
middle.  H  is  the  eyepiece,  J  is  a  hinged  front 
carrying  the  objective  lens,  K,  and  prismatic  lens 
L.  These  are  unnecessary  for  some  work  and  can 
be  replaced  by  a  non-selective  ground  opal  glass. 
The  procedure  in  making  observations  with  this 


Fig.  53.  —  The  F.  E.  Ives  colorimeter. 

instrument  is  obvious.  If  the  colorimeter  readings 
which  are  obtained  from  the  position  of  the  levers 
which  control  the  slit  widths  of  /?,  G,  and  5,  be 
reduced  to  sensation  values  they  become  much  more 
valuable.  H.  E.  Ives  u  has  done  this  in  analyzing 
the  color  of  illuminants,  by  using  the  sensation  curves 
obtained  by  Koenig  and  modified  by  Exner.  These 
are  shown  in  Fig.  54.  They  are  based  upon  experi- 
mental data  which  has  afforded  strong  confirmation 
of  the  Young-Helmholtz  theory  of  color  vision  which 
assumes  three  fundamental  color  sensations  are 
responsible  by  different  degrees  of  excitation  for 


104 


COLOR  AND   ITS  APPLICATIONS 


the  perception  of  all  colors   (#47).     It  is  noted  that 
each  of  the  three  supposed  primary  color  sensations 


1TT 

Blue 


Red. 


Green 


0.39      042      045      045       0.51       0,54      0.57      0.60      0.63      0.66      0.69     0.72 

^ 
Fig.    54.  —  Ko'enig's    sensation    curves. 

is  not  excited  by  a  limited  portion  of  the  spectrum. 
In   fact,   spectral   rays  in    general    are    supposed    to 


Fig.  65.  —  Tri-color  colorimeter  measurements. 

excite  the  sensations  in  relatively  different  degrees, 
depending  upon  the  wave-length.  After  a  prelimi- 
nary investigation  Ives  concludes  that  these  curves 
are  a  much  nearer  approach  to  the  truth  than  those 
obtained  by  Abney.  In  reducing  the  colorimeter 


ANALYSIS   OF  COLOR 


105 


readings  to  sensation  values  it  was  necessary  to 
obtain  the  red,  green,  and  blue  sensation  values  of 
the  colorimeter  screens.  Spectrophotometric  analy- 
sis of  the  screens  combined  with  the  data  in  Fig. 
54  yield  the  primary  sensation  values  of  the  screens 
which  are  obtained  in  relative  values  by  integrating 
the  areas  under  the  sensation  curves  for  the  three 
screens  and  reducing  the  colorimeter  readings  accord- 
ingly. Each  of  the  three  colorimeter  readings  repre- 
sents a  mixture  of  the  three  primary  sensations, 
depending  upon  the  primary  sensation  values  of  the 
colorimeter  screens.  The  procedure  is  simple  but 
more  details,  if  desired,  can  be  obtained  from  the 
original  paper.  The  primary  sensation  values  of 
various  illuminants  compared  with  average  daylight 
as  determined  by  Ives  are  found  in  Table  VIII,  some 
of  which  are  plotted  in  Fig.  55.  The  data  represent 

TABLE  VIII 
Color  of  Illuminants  by  Tri-chromatic  Colorimeter     (See  Fig.  65) 


Source 

Sensation  values 

Red 

Green 

Blue 

1.   Black  body  6000°  abs  
2.  Blue  sky  (S)       ....                   

33.3 
26.8 
32.0 
34.6 
37.7 
64.3 
61.1 
48.6 
48.3 
49.2 
42.5 
45.4 
47.2 
41.0 
29.0 
62.0 
31.3 

33.3 
27.2 
32.0 
33.9 
37.3 
39.5 
40.5 
40.8 
40.8 
40.7 
40.8 
42.0 
41.8 
36.3 
30.3 
37.6 
31.0 

33.3 
46.0 
35.8 
31.5 
25.0 
6.2 
5.4 
10.6 
10.9 
11.1 
16.7 
12.6 
11.0 
22.7 
40.7 
10.5 
37.7 

Blue  sky  (C) 

3.   Overcast  sky  

4.  Afternoon  sun  
5.   Hefner  lamp  ...                                         ... 

6.   Carbon  incandescent  lamp,  3.1  w.  p.  m.  h.  c  
7.  Acetylene 

8.   Tungsten  incandescent  lamp,  1.26  w.  p.  m.  h.  c. 
9.   Nernst  
10.   Welsbach,  \  %  cerium          

11.  Welsbach,  f  %  cerium 

12.  Welsbach,  1  j  %  cerium 

13.  D.  C.  Arc 

14.  Mercury  arc  

15.  Yellow  Flame  arc    

16.   Moore  carbon-dioxide  tube  

106 


COLOR  AND  ITS  APPLICATIONS 


the  means  of  the  values  determined  by  two  methods, 
namely  colorimeter  readings  and  likewise  spectro- 
photometric  data  reduced  to  sensation  values.  (The 
primary  color  sensation  values  of  the  spectral  colors 
and  principal  lines  of  the  cadmium  and  mercury 
spectra  are  plotted  in  Fig.  31.)  The  dotted  line 
represents  the  color  of  a  black  body  (or  an  incandes- 
cent solid  emitting  radiation  non-selectively)  for  tem- 
peratures between  3000  and  7000  degrees  absolute 
(C).  Most  of  the  artificial  illuminants  lie  along  this 
curve.  Those  radiating  selectively  in  the  visible 
spectrum,  such  as  the  yellow  flame  arc  and  Wels- 
bach  mantle,  do  not  lie  upon  it.  Ives  concludes  that 
the  spectral  distribution  of  energy  in  noon  sunlight 
which  reaches  the  earth's  surface  is  quite  similar  to 
that  of  the  black  body  at  5000  degrees  absolute  (C) 
as  computed  from  radiation  laws  (#6). 

s/  Another    instrument   for    tri-color    analysis    which 
is   extremely   simple   is  illustrated  in  Fig.   56.     This 

method,  which  has 
been  applied  by  many 
in  various  color  investi- 
gations, has  been  used 
by  Bloch.12  A  disk  con- 
taining four  circular 

Fig.  56.  —  Arrangement   for   using   color  ,-,  *      . 

filters   before   a   photometer  eyepiece.         apertures,    three    being 

respectively     covered 
by    red,    green,    and    blue    screens,    is    pivoted 


Red 


Green 


Blue 


is  pivoted  so 
that  the  various  screens  can  be  brought  before 
the  ocular  aperture  in  a  photometer  head.  Pho- 
tometric balances  are  made  while  viewing  the  field 
through  the  various  filters  separately  and  the  re- 
sults are  plotted  on  rectangular  coordinates,  the  ratio 
of  red  to  green  intensities  being  plotted  against  the 
ratio  of  blue  to  green  intensities.  Bloch  presents 


ANALYSIS   OF   COLOR  107 

plats  containing  his  color  analysis  of  many  illuminants 
and  the  spectrophotometric  analyses  of  the  filters 
are  also  shown.  Such  results  can  hardly  be  consid- 
ered more  than  approximately  comparative  and  of 
limited  usefulness.  In  general,  data  concerning  the 
color  of  illuminants  or  of  colored  media  obtained 
by  the  tri-color  method  of  analysis  are  limited  in 
usefulness,  owing  to  the  fact  that  the  method  is 
not  sufficiently  analytical.  The  usefulness  of  such 
a  method  is  broader  than  the  tintometer  with  its 
arbitrary  standards  of  color,  but  the  spectropho- 
tometer  and  monochromatic  colorimeter  as  a  rule  yield 
more  useful  data,  the  former  being  'quite  analytical 
for  spectral  examination  and  the  latter  rendering  data 
in  terms  of  the  specific  qualities  of  a  color,  namely, 
hue,  saturation,  and  brightness. 

29.  Other  Methods  of  Color  Analysis.  —  Many 
instruments  have  been  devised  for  color  analysis 
based  on  principles  differing  from  the  foregoing. 
A  number  of  colorimeters  employing  colored  solu- 
tions have  been  used,  the  measurements  usually 
being  made  in  terms  of  the  depth  of  liquids  of  cer- 
tain concentrations.  Purple  and  green  solutions  have 
been  used  by  Fabry  for  eliminating  color  difference 
in  photometry.  In  a  sense  such  a  procedure  is  a 
colorimetric  method  if  it  is  desired  to  use  it  as  such. 
The  Kirchoff-Bunsen  and  Stammer  colorimeters  em- 
ploy colored  solutions  for  the  measurement  of  color. 

Leo  Arons  13  has  devised  a  colorimeter  based 
upon  the  rotation  of  the  plane  of  polarization  by 
quartz  plates  (#  11)  which  have  been  cut  perpendicu- 
larly to  their  crystallographic  axes.  This  instrument 
is  illustrated  in  /,  Fig.  57.  White  light  from  a  dif- 
fusely reflecting  porcelain  disk  is  reflected  into  the 
instrument  through  a  circular  hole,  5,  and  is  rendered 


108 


COLOR  AND   ITS  APPLICATIONS 


plane-polarized  by  the  Nicol  prism,  P.  A  quartz  plate 
at  Q  rotates  the  plane  of  polarization  of  various 
rays  through  various  angles  depending  upon  the 
wave-length.  The  beam  then  passes  through  another 
Nicol  prism,  Aj  thence  through  the  central  portion 
of  the  Lummer-Brodhun  photometer  cube,  W,  and 
to  the  eye  beyond  R.  The  eye  sees  a  circular  patch 
of  light  of  a  certain  color  depending  upon  the  thick- 
ness of  the  quartz  plate  and  the  relative  angular  posi- 
tions of  the  Nicol  prisms.  This  colored  patch  is 


m 


I    BLQ 


Fig.  57. —  Arons  colorimeter. 

matched  in  color  with  the  light  entering  the  side 
tube,  N.  The  latter  beam  is  controlled  in  intensity 
by  the  two  Nicol  prisms,  Pi  and  P2,  and  is  reflected 
by  the  totally  reflecting  prism,  Z>,  to  the  photometer 
cube,  which  in  turn  reflects  the  light  to  the  eye  in 
a  beam  concentric  with  the  first  beam.  The  colored 
media  are  placed  in  front  of  tube  AT,  and  are  pref- 
erably illuminated  by  the  same  source  that  illumi- 
nates the  porcelain  disk  in  front  of  B.  Mixed  colors 
are  obtained,  the  Nicol,  A,  subtracting  certain  rays 
depending  upon  its  angular  position  leaving  the 
remaining  light  colored  instead  of  white.  Six  quartz 
plates  are  provided,  of  thicknesses  0.25,  0.5,  1.0,  2, 
4,  and  8  millimeters  respectively,  which  are  mounted 
in  brass  plates.  These  plates  have  two  identical 


ANALYSIS    OF   COLOR  109 

holes,  one  covered  with  the  quartz  plate,  the  other 
unobstructed.  These  are  arranged  to  slide  in  or 
out  of  the  instrument  at  or  near  Q.  By  sliding  any 
of  the  brass  plates  to  the  side  any  number  of  quartz 
plates  can  be  arranged  one  after  another  and  thus 
the  total  thickness  of  quartz  in  the  path  of  the  beam 
from  B  can  be  adjusted  in  steps  of  0.25  mm.  to  a 
total  thickness  of  15.75  mm.  A  still  greater  variety 
of  colors  can  be  obtained  by  using  two  sets  of  Nicol 
prisms  and  quartz  plates  in  series.  Therefore  the 
tube  in  /  can  be  removed  at  the  plane  xx,  and  tube 
II  connected  at  the  end  yy.  B'  takes  the  place  of 
B  and  lens  U  the  place  of  L.  Any  thickness  of 
quartz  plates  at  Qf  can  be  inserted;  however,  only  a 
single  plate  is  employed  by  Arons,  this  one  being 
3.75  mm.  in  thickness.  In  case  transparent  colored 
media  are  to  be  examined,  a  white  diffusely  reflect- 
ing porcelain  disk  similar  to  the  other  one  is  used  in 
front  of  the  tube  N.  The  two  disks  should  receive 
the  same  intensity  of  illumination  from  the  same 
source.  If  opaque  colors  are  to  be  examined  for 
reflection,  these  are  placed  on  the  porcelain  disk, 
and  the  observer  sees  an  outer  ring  through  R  of  the 
color  of  the  unknown.  This  is  matched  in  color  and 
brightness  by  adjusting  the  thickness  of  quartz  and 
the  angular  position  of  the  Nicol  prisms  until  the 
inner  circle  appears  of  the  same  color  and  brightness 
as  the  outer  ring.  The  measurements  are  recorded 
in  terms  of  the  thickness  of  quartz,  the  angle  between 
A  and  P  and  the  angle  between  Pi  and  P2  and  also 
between  Pf  and  P  if  the  tube  II  is  in  use. 

30.  Templates.  -  -  Much  of  the  early  investigation 
in  color  was  done  with  the  rotating  disks  (Fig.  23) 
and  it  is  quite  natural  that  modifications  of  these 
would  be  made.  Abney  devised  an  ingenious  method 


110  COLOR  AND  ITS  APPLICATIONS 

for  showing  the  effect  upon  the  color  of  the  integral 
light  of  various  spectral  energy  distributions  and  also 
of  showing  that  a  certain  determined  spectrophoto- 
metric  curve  was  in  reality  the  analysis  of  an  integral 
color.  On  determining  the  relative  amounts  of  light 
of  various  wave-lengths  reflected  by  a  pigment  these, 
instead  of  being  plotted  on  rectangular  coordinates 
as  shown  by  the  dotted  lines  in  Fig.  12,  were  plotted 
in  a  special  manner  on  a  disk.  Along 
a  portion,  VR,  of  a  radius  of  the  circle 
in  Fig.  58,  a  wave-length  scale  is  laid 
off.  The  relative  amounts  of  light  of 
different  wave-lengths  reflected  from 
the  pigment  as  determined  by  means 
Fig.  58.— Abney's  of  a  spectrophotometer  are  laid  off  on 
circumferences  of  circles  concentric 
with  the  center  of  the  disk  starting  at  a 
certain  point  of  VR  corresponding  to  the  wave-length. 
The  cardboard  is  now  cut  out  along  the  boundary  line, 
the  template  in  Fig.  58  being  Abney's  template  for  car- 
mine. If  this  disk  be  carefully  adjusted  in  the  plane 
of  a  spectrum  formed  in  space  so  that  various  wave- 
lengths along  VR  coincide  with  corresponding  wave- 
lengths in  the  spectrum  and  the  disk  be  rotated,  on 
combining  the  colored  rays  passing  through  the 
rotating  aperture  upon  a  white  screen  by  means  of  a 
lens  the  color  of  the  integral  light  reflected  from  car- 
mine is  seen.  This  patch  will  be  exactly  like  the 
original  color  in  appearance  providing  the  optical 
parts  of  the  instruments  are  non-selective  and  the 
same  light  is  used  in  producing  the  spectrum  as 
was  used  in  illuminating  the  pigment  when  the 
spectrophotometric  observations  were  made.  Of 
course  the  irrational  dispersion  of  the  prism  must  be 
properly  allowed  for  and  the  spectrum  must  be  narrow. 


ANALYSIS   OF   COLOR 


111 


Instead  of  rotating  the  template  before  an  actual 
spectrum  Abney  used  the  principle  adopted  by  Max- 
well in  his  *  color  box*  (Fig.  52),  thus  rotating  the  disk 
before  a  long  narrow  slit  illuminated  by  the  total 
light  from  the  illuminant.  The  integral  color  was 
viewed  through  the  eyepiece  of  the  spectrometer. 
Abney  made  a  number  of  these  templates  represent- 
ing pigments,  illuminants,  and  the  luminosity  curve 
of  the  eye. 


ELCVATIOH 

Fig.  59.  —  Adaptation  of  Abney's  scheme  for  the  spectroscopic  synthesis  of  color. 

Recently  Ives  and  Brady  14  applied  Abney 's  prin- 
ciple to  the  alteration  of  the  light  from  a  4  w.p.m.h.c. 
carbon  lamp  to  that  of  *  average  daylight'  and  also 
to  that  from  the  blue  sky.  A  Hilger  constant-devia- 
tion spectrometer  was  used,  as  shown  in  Fig.  59. 
The  regular  camera  attachment  B  was  placed  in  the 
position  ordinarily  occupied  by  the  collimator,  the 
latter  being  placed  in  the  position  of  the  objective 
telescope.  The  slit  at  S  is  long  and  narrow  and  is 
illuminated  by  light  from  the  carbon  incandescent 
lamp  reflected  from  a  white  surface  at  F,  the  prin- 
ciple being  the  same  as  just  presented  in  the 
description  of  Abney's  work  on  templates.  These 
templates  were  computed  on  the  assumption  that  the 


112  COLOR  AND  ITS  APPLICATIONS 

relative  spectral  energy  distribution  in  the  spectrum 
of  the  carbon  incandescent  lamp  operating  at  4  watts 
per  mean  horizontal  candle  is  that  derived  from  the 
Wien  equation  (equation  2,  #6)  for  a  black  body  at 
a  temperature  of  2080  deg.  absolute  (C),  and  that  of 
white  light  corresponding  to  a  temperature  of  5000 
deg.  absolute.  The  templates  for  converting  the 
carbon  light  into  blue  sky  light  were  made  from 
relative  spectrophotometric  measurements.  The  disk 
in  position  is  shown  in  the  three  views  of  the  appa- 
ratus taken  from  the  work  cited  above.  The  advan- 
tage of  using  the  templates  before  a  slit  illuminated 
by  white  light  is  that  a  much  greater  amount  of 
light  is  available  than  in  the  case  of  using  it  before 
a  spectrum  and  recombining  the  transmitted  light 
by  means  of  a  lens.  A  comparison  field  can  be 
arranged  by  reflecting  the  light  L  into  the  instrument 
as  shown.  Abney  cut  a  template  corresponding  to 
the  luminosity  curve  of  the  eye  which  is  of  interest, 
but  owing  to  the  work  of  various  modern  investi- 
gators this  has  been  more  accurately  established. 
The  template  scheme  can  be  applied  by  using  disks 
in  which  openings  are  cut  corresponding  to  the  lumi- 
nosity curve  of  the  eye  and  replacing  the  surface 
at  F  before  the  objective  slit  by  a  straight  incandes- 
cent filament;  thus  the  transmissions  of  absorbing 
media  can  be  determined  by  pure  energy  measure- 
ments. 

31.  The  Nutting  Reftectometer.  —  In  the  study 
of  color  it  is  sometimes  desirable  to  ascertain  the 
reflection  coefficients  of  colored  media.  This  can 
be  done  if  the  object  is  diffusely  reflecting  by  means 
of  an  ordinary  brightness  photometer,  although  the 
uncertainties  of  color  photometry  will  be  present  in  any 
case.  However,  Nutting15  has  devised  a  simple  instru- 


ANALYSIS    OF   COLOR 


113 


ment  shown  in  Fig.  60  that  is  very  useful  for  deter- 
mining the  reflection  coefficients  of  any  colored  media 
for  light  incident  from  all  possible  directions  simul- 
taneously. Two  crown  glass 
prisms  of  21  deg.  angle  are 
fastened  over  the  two 
apertures  in  the  end  of  a 
Koenig-Martens  p  o  1  a  r  i  z  a- 
tion  photometer  and  the  lat- 
ter is  inserted  into  a  metal 
ring  which  is  nickel-plated 
and  polished  inside.  The 
light  enters  the  apertures  of 
the  instrument  along  the 
dotted  lines  shown  and  is 
divided  into  two  plane-polar- 
ized beams  by  a  Wollaston 
prism.  These  beams  can 
be  balanced  in  intensity  by 
rotating  the  Nicol  prism.  The  surface  whose  re- 
flection coefficient  is  desired  is  placed  on  one 
side  of  the  ring  completely  covering  it  and  this  is 
illuminated  by  a  non-selective  ground  opal  glass 
on  the  other  side  of  the  ring.  The  instrument  is 
placed  upon  a  wooden  frame  for  convenience.  The 
light  is  reflected  back  and  forth  between  two  planes 
of  *  infinite'  extent  made  practically  so  by  the  polished 
ring.  Simple  theory  shows  that  the  ratio  of  the 
brightness  of  the  unknown  to  that  of  the  ground  opal 
glass  is  a  direct  measure  of  the  reflection  coeffi- 
cient of  the  former  for  the  character  of  the  illumina- 
tion it  receives.  Certain  precautions  must  be  taken 
into  consideration  as  explained  by  Nutting. 

32.   Methods    of    Altering    Brightness    of    Colors 
Non-selectively. — It  is   often  desirable  to  alter  the 


Fig.  60.  —  The  Nutting  reflectom- 
eter. 


114  COLOR  AND   ITS  APPLICATIONS 

brightness  of  colored  lights  without  altering  them 
spectrally.  A  simple  means  is  found  in  varying  the 
distance  of  the  light  source  and  computing  the  rela- 
tive intensities  from  the  *  inverse  square  law.'  How- 
ever, sometimes  this  is  inconvenient.  Sectored  disks 
are  often  resorted  to  with  satisfactory  results.  These 
are  now  being  used  in  photometry  to  a  great  extent, 
the  variable  sectored  disk  devised  by  Hyde  (Fig.  44) 
being  especially  convenient  and  reliable  for  spectro- 
photometry.  The  Brodhun  variable  sector  is  another 
device  very  often  applicable.  In  this  instrument  a 
beam  of  light  is  rotated  and  is  controlled  in  intensity 
by  a  variable  stationary  sector.  Plate  glass  varied 
in  its  angular  position  with  respect  to  the  axis  of  the 
beam  affords  a  means  of  obtaining  a  slight  range  of 
brightnesses,  although  non-selective  glass  is  rarely 
found.  Wire  mesh  and  grids  thoroughly  blackened 
are  satisfactory  in  some  problems.  Neutral  tint 
wedges  have  been  used,  but  it  is  difficult  to  obtain 
strictly  non-selective  smoke  glass.  Ives  and  Luck- 
iesh  1G  studied  the  transmission  characteristics  of 
half-tone  gratings  (black  lines  on  clear  glass)  and 
found  them  to  be  satisfactory  if  properly  used.  Pho- 
tographic screens  are  found  to  serve  some  purposes, 
but  they  must  always  be  calibrated  in  position  owing 
to  their  tendency  to  diffusely  reflect  light.  These 
are  a  few  methods  which  have  proved  helpful  in  the 
proper  place. 

REFERENCES 

1.  Bul.  Bur.  Stds.  1906,  2,  p.  317. 

2.  Astrophys.  Jour.  1912,  25,  p.  239. 

3.  Trans.  I.  E.  S.  1914,  p.  853. 

4.  Phys.  Rev.  1910,  30,  p.  446. 

5.  Bul.  Bur.  Stds.  7,  p.  234. 

6.  Bul.  Bur.  Stds.  1913,  9,  No.  187. 


ANALYSIS   OF   COLOR  115 

7.  Color  Mixture  and  Measurement,  p.  165. 

8.  Trans.  I.  E.  S.  1914,  9,  p.  687. 

9.  Phys.  Rev.  N.  S.  1914,  4,  p.  454. 

10.  Jour.  Franklin  Inst.  July,  Dec.  1907. 

11.  Trans.  I.  E.  S.  1910,  p.  189. 

12.  Electrotech.  Zeit.  1913,  46,  p.  1306. 

13.  L'Industrie  Elec.  July  25, 1911. 

14.  Jour.  Franklin  Inst.,  178,  p.  89. 

15.  Trans.  I.  E.  S.  1912,  7,  p.  412. 

16.  Phys.  Rev.  1911,  32,  p.  522. 


CHAPTER   VI 
COLOR  AND  VISION 

33.  The  Eye.  —  Color  vision  is  not  essential, 
because  achromatic  vision  serves  the  totally  color- 
blind person  well.  However,  'the  ability  to  perceive 
colors  extends  the  usefulness  of  the  sense  of 
sight  very  much.  It  not  only  adds  greatly  to  our 
pleasure  but  is  utilized  in  many  ways.  The  eye 
can  be  considered  optically  as  a  rather  simple  instru- 
ment, as  indicated  by  the  photograph  of  the  middle 
vertical  section  of  a  human  eye  shown  in  Fig.  61. 
It  is  seen  that  the  refracting 

Sj^^^^^ .  « 

V%v          media    consist    of   the    cornea, 
>J^^      ik       aqueous  humor,  lens,  and  vitre- 
£&     pb      Bt     ous  humor.    The  retina,  which 
I     consists     of    the     optic    nerve 
ifr  Jilf      spread    out    over    the    interior 
of  the  eyeball,   is   a  very  thin 
^B  ^jjr  membrane,    and    can   be    seen 

in  the  illustration  partially  de- 

F1kfh^nV^ealSeCti0n0f  Cached     from    the    wall.     The 

radii    of    curvature,    thickness, 

and  refractive  index  of  the  various  eye  media  as 
determined  by  Helmholtz  l  are  given  in  Table  IX. 
The  normal  eye,  while  being  a  wonderfully  adapt- 
able instrument,  is  not  free  frpm  errors,  owing 
to  the  fact  that  it  is  optically  quite  simple.  The 
chief  error  of  interest  here  is  its  lack  of  achromatism. 
If  an  image  of  an  object  illuminated  by  light  having 
a  continuous  spectrum  be  produced  by  a  simple  lens 

116 


COLOR  AND  VISION 


117 


TABLE   IX 

Optical  Constants  of  the  Eye 


Distant 
vision 

Near 
vision 

(15  cm.) 

Index  of  refraction  of  the  humors  and  cornea  
Index  of  refraction  of  the  crystalline  lens  , 

1.3365 
1.4371 

Effective  index  of  refraction  of  lens  surrounded  by  humors 
Radius  of  outer  surface  of  cornea  
Radius  of  first  lens  surface  
Radius  of  second  lens  surface  

1.0753 
7.8  mm. 
10.0 
6.0 

7.8  mm. 
6.0 
5.5 

Thickness  of  cornea  
Thickness  of  crystalline  lens  
Distance  of  first  lens  surface  from  cornea 

0.4 
3.6 
3.6 

0.4 
4.0 
3.2 

Distance  of  second  lens  surface  from  cornea  

7.2 

7.2 

it  will  be  found  to  have  a  red,  blue,  or  purple  fringe. 
This  is  readily  understood  from  Fig.  62,  which  repre- 
sents a  schematic  eye  in  which  only  the  simple  lens 
is  considered.  Owing  to  the  difference  in  the  refrac- 
tive index  of  a  medium  for  rays  of  different  wave- 
length, such  a  result  as  is  exaggerated  in  Fig.  62  will 
obtain.  The  refractive  index  being  greater  for  rays 
of  shorter  wave-length,  the  blue  rays  will  be  deviated 
or  refracted  more  than  the  yellow  rays,  and  the 
latter  more  than  the  red  rays.  Naturally  the  eye 
focuses  for  the  brightest  rays,  which  in  ordinary 
light  are  the  yellow-green  or  yellow  rays.  There- 
fore, the  blue  and  red  rays  will  be  out  of  focus, 
with  the  result  that  the  image  of  the  point,  P,  will 
be  surrounded  by  a  purple  fringe.  This  is  of  im- 
portance in  vision,  as  will  be  shown  later.  The  lack 
of  achromatism  of  the  eye  can  be  demonstrated  very 
simply.  On  viewing,  by  reflected  light,  the  concentric 
circles  shown  at  the  right  of  Fig.  62  held  close  to 
the  eye  they  appear  colored.  A  very  striking  experi- 
ment is  found  in  focusing  a  line  spectrum  —  that 
of  mercury  will  suffice  —  upon  a  ground  glass.  On 


118  COLOR  AND  ITS  APPLICATIONS 

viewing  it  at  a  normal  distance  (14  inches),  the  yellow 
and  green  lines  will  appear  sharply  focused,  but  the 
blue  and  violet  lines  will  appear  hazy  and  quite  out 
of  focus.  On  bringing  the  eye  closer  the  latter  lines 
will  begin  to  appear  clearer,  and  finally,  when  the  eye 
is  within  about  six  inches  of  them,  they  will  still 
appear  clear-cut,  while  it  will  be  quite  impossible  to 
accommodate  the  eye  sufficiently  to  focus  the  yellow 
and  green  lines.  In  other  words  the  eye  is  near- 
sighted (myopic)  for  blue  rays  and  far-sighted  (hyper- 
opic)  for  red  rays.  On  viewing  a  narrow  continuous 
spectrum  at  some  distance  the  blue  end  appears  to 


Fig.  62.  —  Showing  the  effect  of  chromatic  aberration  in  the  eye. 

flare  out.  Another  simple  demonstration  is  found 
in  viewing  an  illuminated  slit  through  a  dense  cobalt 
glass  which  transmits  extreme  red  and  violet  rays. 
On  accommodating  the  eye  for  a  point  behind  the  slit 
a  red  image  with  a  violet  halo  is  seen.  On  accommo- 
dating for  *a  point  in  front  of  the  slit  a  violet  image 
with  a  red  halo  is  seen.  This  defect  plays  a  promi- 
nent, though  usually  unnoticed,  part  in  vision.  A 
lens  can  be  made  practically  achromatic  by  combining 
a  convergent  lens  of  crown  glass  with  a  divergent  lens 
of  flint  glass.  The  former  is  more  strongly  conver- 
gent for  blue  than  for  red  rays,  while  the  latter  is 
more  strongly  divergent  for  blue  than  for  red  lights. 
It  is  thus  possible  to  bring  the  red  and  blue  rays  in 
coincidence  at  a  focus.  Inasmuch  as  it  is  only  pos- 
sible to  bring  two  rays  exactly  into  coincidence  by 


COLOR  AND  VISION  119 

a  two-piece  lens,  such  a  lens  is  not  truly  achromatic, 
though  practically  so  for  most  purposes.  By  com- 
bining more  lenses  the  approach 
to  true  achromatism  is  brought  /c\  Fi 

as  near  as  desired.     A  simple  /     \ 

achromatic  lens  is  illustrated  in 
Fig.  63. 

The  retina  has  been  found 
to  vary  in  its  sensibility  to  colors. 
The  central  region  is  sometimes 
known  as  the  yellow  spot,  be- 
cause it  apparently  absorbs  the  ^63. -A  simple  achromatic 

violet  and  blue  rays  to  a  greater 

degree  than  other  rays.  The  effect  of  the  yellow  spot 
is  often  seen  in  viewing  colors  one  after  another,  and 
it  is  quite  noticeable  at  twilight  illumination.  It 
appears  of  somewhat  irregular  outline  in  after- 
images. Studies  of  the  various  zones  of  the  retina 
as  to  their  sensibility  to  various  colors  yield  results 
in  general  similar  to  those  shown  in  Fig.  64.  The 
center  of  the  fovea  corresponds  to  the  center  of 
the  circle.  The  solid  line  shows  the  boundary  for 
the  perception  of  light.  The  visual  field  for  one 
eye  extends  outward  about  90  deg.  from  the  normal 
optical  axis  of  the  eye,  inward  about  60  deg.,  down- 
ward 70  deg.,  and  upward  50  deg.  The  dashed  line 
represents  the  extreme  limits  where  blue  can  be 
perceived  as  such  and  the  remaining  two  lines  repre- 
sent respectively  the  limits  for  red  and  green  per- 
ception. These  facts  must  be  reconciled  with  any 
satisfactory  theory  of  vision.  It  might  be  noted  here 
that  each  eye  has  a  blind  spot  —  the  point  of  entrance 
of  the  optic  nerve — which  is  totally  insensitive  to  light. 
The  retina,  which  consists  of  the  optic  nerve  spread 
out,  is  covered  with  a  mass  of  microscopic  'rods'  and 


120 


COLOR  AND  ITS  APPLICATIONS 


'cones'  (#48)  projecting  outward  toward  the  lining  of 
the  eyeball  which  play  an  important  part  in  theories 
of  vision. 

34.  Brightness  Sensibility.  —  The  sensibility  of 
the  retina  to  brightness  differences  is  greatest  over 
a  wide  range  of  intensities,  falling  off  at  extremely 


10 


120 


135 


22 


315 


300 


Colorless 


..  Red 
-  Green 


Blue  

Fig.  64.  —  Limits  of  the  visual  field  for  colored  and  colorless  lights. 

low  and  extremely  high  brightnesses.  With  decreas- 
ing intensities  the  sensibility  diminishes  more  rap- 
idly for  rays  of  longer  wave-length  than  for  those  of 
shorter  wave-length.  Koenig  and  Brodhun  2  have 
done  excellent  work  in  this  field  as  well  as  in  many 
other  fields  pertaining  to  vision.  They  determined 
the  least  perceptible  brightness  increment  for  lights 


COLOR  AND  VISION 


121 


of  various  colors  including  white,  for  brightnesses  of 
a  neutral  tint  surface  (' white')  illuminated  to  various 
intensities  from  1,000,000  meter-candles  to  nearly  the 


-5 


-2 


-I  0  12  3 

LOGARITHM  OF  ILLUMINATION,  I 


Fig.  65.  — Brightness  sensibility  data.     (See  Table  X.) 

threshold  of  vision,  using  an  artificial  pupil  of  1  sq. 
mm.  area.  They  started  at  600  meter-candles  and 
extended  the  illumination  above  and  below  by  the 
various  steps  indicated  in  the  accompanying  table. 
The  data  for  Koenig's  eye  after  modification  by 
Nutting3  are  shown  in  Fig.  65  and  Table  X.  Koenig 
and  Brodhun  did  not  include  the  increment  (5B) 
in  the  total  brightness  (B)  in  calculating  the  values 
dB/B.  Nutting  recomputed  the  data  with  the  thresh- 
old value  included.  It  is  seen  that  the  increment  of 
brightness  difference  just  perceptible,  increases  as 
the  brightness  decreases  and  more  rapidly  for  the 
rays  of  longer  wave-length.  At  high  illuminations 
the  minimal  perceptible  increment  is  about  the  same 
(1.6%)  for  all  colors,  including  white.  For  the  ordi- 
nary range  of  brightnesses  5B/B,  is  constant,  which 
fact  is  known  as  Fechner's  law,  and  the  constant  is 
called  Fechner's  coefficient. 


122 


COLOR  AND  ITS  APPLICATIONS 


TABLE   X 

Data  of  Koenig  and  Brodhun  on  Brightness  Sensibility 
Recalculated  by  Nutting 


Wave- 
length = 

B0  = 

=  0.670/x 
=  0.060 

0.605/i 
0.0056 

0.575M 
0.0029 

0.505M 
0.00017 

0.470M 
0.00012 

0.430M 
0.00012 

Meter 
Candles 

8B 

B 

200,000 

0.0425 

100,000 

0.0241 

0.0325 

50,000 

0.0210 

0.0255 

0.0260 

20,000 

0.0160 

0.0183 

0.0205 

0.0195 

10,000 

0.0156 

0.0163 

0.0179 

0.0181 





5,000 

0.0176 

0.0158 

0.0166 

0.0160 

2,000 

0.0165 

0.0180 

0.0180 

0.0175 

0.0180 



1,000 

0.0169 

0.0198 

0.0185 

0.0184 

0.0167 

0.0178 

500 

0.0202 

0.0235 

0.0180 

0.0194 

0.0184 

0.0214 

200 

0.0220 

0.0225 

0.0225 

0.0220 

0.0215 

0.0245 

100 

0.0292 

0.0278 

0.0269 

0.0244 

0.0225 

0.0246 

50 

0.0376 

0.0378 

0.0320 

0.0252 

0.0250 

0.0272 

20 

0.0445 

0.0460 

0.0385 

0.0295 

0.0320 

0.0345 

10 

0.0655 

0.0610 

0.0582 

0.0362 

0.0372 

0.0396 

5 

0.0918 

0.103 

0.0888 

0.0488 

0.0464 

0.0494 

2 

0.1710 

0.167 

0.136 

0.0655 

0.0715 

0.0600 

1 

0.258 

0.212 

0.170 

0.0804 

0.0881 

0.0740 

0.5 

0.376 

0.276 

0.208 

0.0910 

0.096 

0.0966 

0.2 

0.332 

0.268 

0.110 

0.127 

0.116 

0.10 

0.396 

0.133 

0.138 

0.137 

0.05 

0.183 

0.185 

0.154 

0.02 

0.251 

0.209 

0.223 

0.01 

0.271 

0.189 

0.249 

0.005 

0.325 

0.300 

0.312 

0.002 

0.369 

The  value  of  the  minimal  perceptible  increment 
depends  largely  upon  the  method  of  making  the 
measurements.  Usually  the  brightness  of  one  of  the 
two  parts  of  the  photometric  field  is  varied  until  it 
appears  just  perceptibly  brighter  or  darker  than  the 
comparison  field.  This  procedure  yields  values  of 


COLOR  AND  VISION  123 

the  least  perceptible  increment  comparable  with  the 
foregoing  value.  In  precision  photometry  the  accu- 
racy is  often  as  high  as  0.1  per  cent;  however,  another 
factor  enters  into  such  procedure.  The  brightness 
of  one  part  of  the  field  is  varied  between  certain 
limits  at  which  it  is  respectively  distinctly  brighter 
and  darker  than  the  comparison  field,  and  these  limits 
are  gradually  brought  nearer  together  until  finally 
an  attempt  is  made  to  estimate  the  middle  point. 
This  cannot  be  considered  a  measure  of  brightness 
sensibility.  However,  P.  W.  Cobb  has  employed  a 
method  which  is  of  considerable  interest  here  inas- 
much as  he  obtains  values  for  the  minimal  percep- 
tible increment  for  white  light  smaller  than  0.5  per 
cent.  In  these  experiments  the  test  field  was  exposed 
to  the  view  of  the  observer  for  a  brief,  but  constant, 
period,  after  which  his  judgment  was  recorded.  One 
side  of  the  field  appeared  either  brighter  or  darker, 
or  no  difference  in  brightness  was  distinguishable. 
This  procedure  was  repeated  for  a  range  of  aspects 
of  the  test  field  varying  from  that  in  which  one  side 
appeared  distinctly  darker  for  a  number  of  succes- 
sive exposures  to  that  in  which  it  appeared  definitely 
brighter.  Obviously,  by  progressing  in  small  steps 
between  these  two  limits  (presenting  these  various 
aspects  in  haphazard  order)  there  were  several  near 
equality  where  the  judgment  was  uncertain.  After 
reducing  the  data  by  a  special  method  Cobb  con- 
cludes that  the  minimal  perceptible  increment  is 
much  smaller  than  that  obtained  by  Koenig  and 
Brodhun. 

The  data  of  Koenig  and  Brodhun  has  been  ex- 
tended by  Nutting  by  computation  to  the  point  where 
65/5  =  1;  that  is,  to  the  threshold  value.  This 
computation  is  very  interesting,  though  perhaps  not 


124  COLOR  AND   ITS  APPLICATIONS 

entirely  free  from  criticism.  BQ  in  Table  X  repre- 
sents the  threshold  value  of  brightness  measured  as 
a  fraction  of  the  standard  high  brightness.  Brightness 
By  is  proportional  to  illumination,  7,  and  inasmuch 
as  it  is  a  brightness  that  is  perceived  the  symbol  B 
is  used. 

35.  Hue  Sensibility.  —  Notwithstanding  the  fact 
that  the  visible  spectrum  is  generally  considered  to 
exhibit  only  six  or  seven  colors,  four  of  which,  red, 
yellow,  green,  and  blue  are  strikingly  distinctive, 
there  are  theoretically  present  an  infinite  number 
of  hues.  The  number  of  distinct  hues  that  a  person 
is  able  to  distinguish  depends  upon  the  manner  in 
which  the  experiment  is  conducted.  Edridge-Green 4 
states  that  he  has  *  never  met  with  a  man  who  could 
see  more  than  29  monochromatic  patches  in  the 
spectrum.'  Rayleigh,5  who  is  able  to  detect  the  dif- 
ference in  hue  of  the  sodium  D  lines  (0.5890/x  and 
0.5896/0,  could  distinguish  only  17  hues  on  Green's 
apparatus,  and  claims  this  is  due  to  the  method  of 
comparing  the  patches.  In  Green's  apparatus  the 
principle  is  that  of  two  opaque  screens  held  over  a 
spectrum  and  slightly  separated  from  each  other. 
One  is  then  moved  until  the  hue  at  its  edge  appears 
different  from  that  at  the  edge  of  the  other.  With 
an  apparatus  employing  the  principle  of  the  Maxwell 
color  box  Rayleigh  was  able  to  distinguish  many  more 
hues.  By  the  use  of  spectral  apparatus  as  high  as 
128  distinctly  different  spectral  hues  have  been  seen. 
It  is  not  difficult  to  obtain  by  the  use  of  dyed  media 
a  series  of  25  distinct  spectral  hues.  Ridgeway,6 
by  beginning  with  papers  dyed  to  represent  six 
spectral  hues  and  adding  various  intermediate  hues, 
obtained  36  distinct  hues.  The  data  on  hue  sensi- 
bility vary  considerably,  which  perhaps  is  due  to 


COLOR  AND  VISION 


125 


variations  in  the  refinement  and  nature  of  the  experi- 
mental methods   employed. 

Some  excellent  data  have  been  obtained  by  Steind- 
ler  7  on  hue  sensibility  for  twelve  subjects.  The  posi- 
tions of  the  maxima  differed  somewhat  for  the  various 


\ 


0.40         044          046          0.5Z          056          0.60          0.64 
^U.  WAVE  LENGTH 

Fig.  66.  — Hue  sensibility.     (Steindler's  Eye.) 


observers.  The  hue  sensibility  curve  for  Steindler's 
eye  is  shown  in  Fig.  66  and  the  mean  positions  of 
the  maxima  and  minima  of  the  hue  sensibility  curves 
for  the  twelve  observers  and  the  wave-length  limen 
of  'just  perceptible  difference'  are  given  in  Table  XI. 
Nutting8  has  used  the  mean  results  obtained  by 
Steindler  in  deriving  a  natural  scale  of  color.  These 
mean  results,  including  Nutting's  color  scale,  are  plotted 
in  Fig.  67.  The  hue  sensibility  curve,  S,  was  plotted 
by  connecting  the  mean  positions  of  the  minima  and 


126 


COLOR  AND   ITS  APPLICATIONS 


TABLE   XI 

Steindler's  Data  on  Hue  Sensibility 
(The  mean  for  twelve  eyes) 


Position 

Perceptible  limen 

First  maximum                                    .            .    . 

0.456M 

0.0293^ 

Second  maximum 

0.534 

0.0334 

Third  maximum  

0.621 

0.0375 

First  minimum                         

0.440 

0.0247 

Second  minimum                                           -  -  - 

0.492 

0.0136 

Third  minimum 

0.681 

0.0139 

Fourth  minimum                

0.635 

0.0300 

maxima  for  the  twelve  observers  with  smooth  curves. 
The  limen  (least  perceptible  difference  in  terms  of  M) 
curve,  L,  is  plotted  in  the  same  manner.  For  the 


24 


20 


16 


12 


0.044 
0.040  ^ 
0.036  -A 
0.032  t 
0.02.8  § 
0.024  5 
0.020  ^ 
0.016  < 
0.01  2  £ 
0.006  § 
0.00.4 
n 

X 

x. 

\ 

s 

s^ 

\ 

/ 

V 

1 

\ 

lr 

: 

\ 
i 

\ 

c 

A 

i 

\ 

\ 

i 

• 

\ 

1 

V     / 

\^ 

1 

\ 

/ 

\ 

i 

/ 

V 

> 

1 

/ 

\ 

A 

A 

y 

^ 

i 

y 

^~ 

--- 

^s 

/ 

i^ 

s 

\ 

s~ 

•\^ 

/ 

\ 

\. 

*s~ 

\ 

^ 

^x 

N 

0.6Z      0.66 
Fig.  67. — Hue  sensibility,  limen,  and  color  scale. 


>  WAVE  UMGTH 


details  of  the  procedure  adopted  in  obtaining  the 
color  curve,  C,  the  reader  is  referred  to  the  original 
paper.  A  difference  of  one  unit  in  the  color  scale 
represents  a  difference  in  color  that  is  just  easily 
perceptible.  It  will  be  noted  that  the  color  curve 


COLOR  AND  VISION  127 

indicates  there  are  22  of  these  colors  'just  easily 
perceptible'  within  the  spectral  limits  shown. 

36.  Saturation  Sensibility.  —  The  data  on  the 
sensibility  of  the  eye  to  changes  in  saturation  are 
not  very  extensive  or  definite.  Nutting  9  states  that 
with  his  monochromatic  colorimeter  the  probable 
error  in  the  'per  cent  white'  observations  on  a  nearly 
spectral  matte  orange  pigment  was  about  ten  per 
cent.  L.  A.  Jones  10  claims  an  accuracy  of  the  order 
of  three  per  cent  for  the  'per  cent  white'  readings 
(Table  VII)  for  this  monochromatic  colorimeter  of 
improved  type.  The  accuracy  of  course  will  vary 
with  the  hue,  brightness,  and  degree  of  saturation 
of  the  colors.  H.  Aubert  n  determined  the  smallest 
sector  of  color  that  would  be  just  apparent  on  a  rotat- 
ing white  disk  to  be  2  or  3  degrees  —  less  than  one 
per  cent.  With  black  and  gray  disks  he  found  that 
even  smaller  sectors  were  recognized.  His  experi- 
ments on  the  differential  limen  of  color  sensitivity 
indicated  that  on  a  black  background  the  stimulus- 
increments  for  orange,  blue,  and  red  were  respectively 
0.95,  1.54,  and  1.67  per  cent  in  order  to  produce  a 
just  noticeable  increase  in  saturation. 

Geissler 12  studied  the  problem  whether  the  number 
and  sizes  of  the  colored  stimulus-increments  corre- 
sponding to  just  noticeable  saturation  differences 
would  lend  themselves  to  a  measure  of  saturation. 
The  problem  was  attacked  from  two  extremes;  one 
by  gradually  reducing  a  maximally  saturated  pigment 
color,  and  the  other  by  introducing  more  and  more 
color  into  a  colorless  stimulus.  He  employed  the 
rotating  double  color  disk  with  the  Zimmerman  col- 
ored and  gray  papers  illuminated  with  an  artificial 
daylight  devised  by  Ives  and  Luckiesh.  In  the  first 
method  he  used  red  beginning  with  maximal  sat- 


128  COLOR  AND   ITS  APPLICATIONS 

uration  —  360  degrees  of  red  —  for  both  the  inner 
and  outer  concentric  components  of  the  double  disk 
and  gradually  added  small  amounts  of  gray  (of  the 
same  brightness  as  the  red  as  measured  with  a 
flicker  photometer)  to  the  inner  or  smaller  disk  until 
it  appeared  just  perceptibly  less  saturated  than  the 
outer  or  larger  disk.  This  procedure  was  then  re- 
versed, the  outer  disk  being  decreased  in  saturation 
until  the  change  was  just  perceptible  as  compared 
with  the  inner  disk  whose  saturation  was  kept  con- 
stant. This  was  done  for  seven  different  degrees  of 
saturation,  ranging  from  360°  of  red  to  110°  of  red 
plus  250°  of  gray  of  the  same  brightness  as  measured 
by  the  flicker  photometer.  His  results  indicate  that 
the  stimulus-increments  corresponding  to  just  notice- 
able saturation-differences  are  approximately  con- 
stant (about  4°  of  gray)  at  such  different  stages  of 
saturation  as  325°  red  plus  35°  gray,  230°  red  plus 
130°  gray,  and  110°  red  plus  250°  gray.  Geissler 
states  that  'it  seems  fair  to  assume  that  the  incre- 
ment-values would  have  remained  constant  at  the 
intervening  stages  and  perhaps  also  at  a  stage  not 
far  removed  from  the  absolute  color-limen,'  which 
latter  averaged  for  the  four  observers  with  the  red 
paper  about  1.2°.  That  is,  a  sector  of  1.2°  of  red 
when  mixed  with  358.8°  gray  causes  a  just  percep- 
tible appearance  of  color.  It  appears  from  the  fore- 
going that  the  estimated  number  of  least  perceptible 
differences  in  saturation  of  the  red  pigment  under 
the  conditions  of  the  experiment  is  about  100. 

Another  group  of  experiments  was  made  with 
nine  observers  using  red,  yellow,  green,  and  blue 
colored  papers  and  their  corresponding  grays.  These 
measurements  were  made  for  each  eye  separately 
and  for  binocular  vision.  Geissler  places  no  great 


COLOR  AND  VISION  129 

emphasis  upon  the  absolute  values  of  the  results 
because  of  the  lack  of  sufficient  observers  and  the 
incompleteness  of  the  investigation  at  present.  How- 
ever, it  is  of  interest  to  give  the  mean  results  for  the 
nine  observers.  The  averages  for  binocular  vision 
were,  as  a  rule,  lower  than  for  monocular  vision. 
The  results  for  all  observers  for  monocular  and  binoc- 
ular vision  gave  as  the  mean  limenal  values  of  color 
saturation  for  red,  yellow,  green,  and  blue  respec- 
tively, 2.23°,  5.81°,  7.19°,  and  2.99°.  That  is,  these 
values  represent  the  smallest  increments  required 
to  distinguish  between  *  color  and  no  color/  The 
comparison  was  made  between  a  gray  disk  and  a 
concentric  disk  of  the  same  gray  in  which  the  color 
was  introduced.  The  brightnesses  were  previously 
equated  by  means  of  a  flicker  photometer.  The 
colored  papers  differed  from  each  other  in  brightness 
and  saturation,  which  appeared  to  have  an  influ- 
ence on  the  values  of  just  perceptible  saturation-dif- 
ference. Since  the  green  requires  a  limen  three 
times  as  great  as  that  of  red  it  appears  to  Geissler 
that  it  is  reasonable  to  assume  that  its  saturation  is 
only  one-third  as  great  as  the  red  and  about  one- 
half  that  of  the  blue.  These  figures  agree  approxi- 
mately with  a  number  of  estimates  of  saturations 
made  by  some  of  the  observers,  but  in  the  absence 
of  sufficient  data  little  emphasis  is  given  to  this  point. 
Experiments  with  a  practically  color-blind  subject 
indicated  that  his  limenal  values  were  extremely 
high,  being  37°,  18°,  140°,  and  8.25°  respectively  for 
the  red,  yellow,  green,  and  blue  papers.  No  analysis 
of  his  defect  was  made. 

There  appears  to  be  a  need  for  a  further  explora- 
tion in  this  interesting  field. 

37.    Visual  Acuity  in  Lights  of  Different  Colors.  — 


130  COLOR  AND   ITS  APPLICATIONS 

As  has  already  been  shown  the  eye  is  not  achromatic; 
that  is,  rays  differing  in  wave-length  do  not  come  to 
a  focus  at  the  same  point,  with  the  result  that  the 
image  of  an  object  illuminated  by  light  of  extended 
spectral  character  is  not  sharply  defined  upon  the 
retina.  Louis  Bell  13  compared  the  acuity  of  the  eye 
or  its  ability  to  distinguish  fine  detail  in  tungsten 
and  mercury  arc  lights  and  obtained  results  indi- 
cating an  advantage  for  the  latter  illuminant.  This 
he  attributed  to  the  more  nearly  monochromatic 
light  emitted  by  the  mercury  arc.  It  will  be  remem- 
bered (Fig.  4)  that  the  preponderance  of  visible  rays 
is  confined  to  a  rather  narrow  wave-length  range  in 
the  yellow  and  green  regions  of  the  mercury  spectrum. 
The  author  14  verified  these  results  and  extended 
the  investigation  to  lights  of  the  same  color  but  dif- 
fering in  spectral  character.  By  using  the  lights 
whose  spectra  are  shown  in  Fig.  17,  no  difficulties 
of  color  photometry  were  encountered.  Screens  &, 
c,  d,  used  with  a  vacuum  tungsten  lamp  operating 
at  7.9  lumens  per  watt  yielded  lights  of  the  same 
yellow  color  but  of  different  spectral  character.  Like- 
wise screens  e  and  /  yielded  two  green  lights,  one 
purely  monochromatic  (mercury  green  line),  and  the 
other  a  green  of  extended  spectral  character.  The 
data,  except  in  case  4,  Table  XII,  were  not  obtained 
as  usual  by  using  fine  detail  at  the  limit  of  discrimi- 
nation but  instead,  hi  terms  of  equal  '  readability ' 
of  a  page  of  type,  which  proved  after  some  practise 
to  be  a  rather  definite  criterion.  Some  such  method 
should  be  applicable  to  many  practical  investiga- 
tions in  lighting,  for  it  renders  results  in  terms  of 
a  criterion  which,  although  apparently  indefinite,  is 
found  to  be  quite  definite  and  one  which  renders 
results  full  of  significance.  The  results  for  the 


COLOR  AND  VISION 


131 


TABLE   XII 
Relative  Illumination  for  Equal  Readability 


Case 

Source 

Screen 

Color 

Approx. 
foot  candles 

Relative 
illumination 

1 

Mercury  arc 
Tungsten  lamp 

f 
e 

green  line 
green 

2.0 

1.00 
1.75 

2 

Tungsten  lamp 
Tungsten  lamp 

d 
c 

yellow 
yellow 

4.0 

1.00 
1.33 

3 

Sodium  lines 
Tungsten  lamp 

none 
c 

yellow  lines 
yellow 

0.5 

1.00 
1.66 

4 

Mercury  arc 
Tungsten  lamp 

f 
e 

green  line 
green 

0.6 

1.00 
5.10 

author's  eye  are  shown  in  Table  XII  and  are  given 
in  terms  of  the  relative  illumination  required  for 
equal  readability  of  a  page  of  type.  In  case  4  an 
acuity  object  proposed  by  H.  E.  Ives  l5  and  devel- 
oped by  P.  W.  Cobb  was  used.  Here  the  criterion 
was  the  ability  to  perceive  fine  lines  at  the  limit  of 
discrimination. 

Other  observers  obtained  results  of  a  similar 
nature  with  the  same  apparatus.  No  stress  is  laid 
upon  the  accuracy  of  the  absolute  values,  but  it  is 
conclusively  evident  that  monochromatic  light  is 
superior  for  discriminating  fine  detail.  Later  it  was 
shown,  16  as  was  expected  from  the  foregoing,  that 
monochromatic  light  was  superior  to  daylight  for 
discriminating  fine  detail.  In  this  case  the  Ives 
acuity  object  was  viewed  against  a  white  magnesium 
oxide  surface  which  was  illuminated  to  an  intensity 
of  10  meter  candles  (approximately  one  foot  candle). 
The  visual  acuity  on  the  Snellen  scale  was  found  to 
be  1.28  and  1.11  respectively  for  daylight,  and  mono- 


132  COLOR  AND   ITS  APPLICATIONS 

chromatic  green  light  of  equal  intensities  and  results 
for  tungsten  light  and  daylight  were  practically  iden- 
tical. Another  experiment  showed  that  for  visual 
acuity  of  1.28  on  the  Snellen  scale  the  intensity  of 
illumination  with  daylight  or  tungsten  light  was 
nearly  three  times  that  required  for  the  same  visual 
acuity  with  monochromatic  green  light.  As  the 
brightness  of  the  background  was  increased  it  ap- 
peared that  the  difference  in  visual  acuity  under  a 
given  illumination  of  tungsten  light  and  monochro- 
matic light  decreased. 

The  superior  defining  power  of  monochromatic 
light  having  been  demonstrated,  it  is  of  interest  to 
learn  if  there  is  any  difference  in  the  defining  power 
of  monochromatic  lights  of  different  colors.  Dow  17 
measured  visual  acuity  in  light  of  different  colors 
using  electric  lamps  screened  with  colored  media 
and  arrived  at  the  conclusion  that  the  blue-green 
region  of  the  spectrum  showed  greater  defining 
power.  Ashe  ls  used  red,  green,  blue  and  clear 
glasses  with  incandescent  lamps  and  found  visual 
acuity  least  for  the  red  and  increasing  in  the  order 
green,  blue  and  clear  glass  for  the  same  illumina- 
tion; however,  the  data  were  too  incomplete  to  war- 
rant any  definite  conclusions.  Loeser 19  used  red, 
green,  and  white  papers  on  which  black  characters 
were  printed.  The  papers  were  brought  to  equal 
brightness  and  visual  acuity  was  determined  by  noting 
the  greatest  distance  at  which  the  observer  could 
distinguish  the  details  on  the  papers.  He  found 
acuity  greater  for  green  light  than  for  red  light,  and 
also  that  the  characters  on  the  white  card  could  be 
distinguished  at  nearly  as  great  a  distance  as  those 
on  the  green  card.  A  serious  defect  in  this  method 
is  the  fact  that,  the  distances  not  being  constant,  the 


COLOR  AND  VISION 


133 


change  required  in  the  accommodation  of  the  eye 
complicates  the  results.  Uhthoff  20  determined  visual 
acuity  in  monochromatic  lights  of  different  wave- 
lengths, but  gives  no  data  on  the  relative  brightnesses 
of  the  colored  lights.  A  serious  defect  in  most  of 
the  above  work  is  the  fact  that  the  lights  were  neither 
monochromatic  nor  did  their  spectra  extend  over 
equal  ranges  of  wave-lengths.  The  same  criticism 
is  applicable  to  the  work  of  Rice,21  who  performed  an 
extensive  investigation  of  the  problem. 

In    order    to    determine    visual    acuity    in    mono- 
chromatic lights  of  different  colors  at  ordinary  bright- 


Fig.  68.  —  Apparatus  for  determining  visual  acuity  in  monochromatic  lights. 

nesses,  the  author  22  devised  the  apparatus  shown 
diagrammatically  in  Fig.  68.  The  lines  of  the  acuity 
object,15  c,  having  a  highly  illuminated  ground  glass 
background,  J,  were  focused  crosswise  on  the  slit 
of  a  Hilger  wave-length  spectrometer  by  the  lens,  /. 
On  looking  into  the  eyepiece  these  lines  were  viewed 
against  a  background  whose  color  depended  upon  the 
position  of  the  prism,  the  wave-length  being  indicated 
on  the  drum,  n.  On  the  pointer  in  the  eyepiece  was 
mounted  a  minute  piece  of  magnesium  sulphate,  mm, 
at  an  angle  leaning  away  from  the  eye  at  the  top. 
This  was  illuminated  by  means  of  the  frosted  tung- 


134  COLOR  AND   ITS  APPLICATIONS 

sten  lamp,  jy  the  light  being  reflected  downward  by 
the  mirror,  o.  Slides  hh  controlled  the  width  of  the 
photometric  field,  and  an  artificial  pupil,  &,  was  placed 
in  front  of  the  eyepiece.  The  drum,  &,  controlled 
by  means  of  a  belt  the  size  of  the  lines  of  the  test 
object  which  was  read  from  drum  e.  The  photo- 
metric balance  was  made,  in  the  case  of  each  mono- 
chromatic light  used,  by  balancing  it  against  the 
white  surface  mm,  the  lines  of  the  acuity  object  at 
the  time  being  too  small  to  be  visible.  A  feature 
of  this  acuity  object  which  is  essential  for  such  a 
use  is  that  the  average  brightness  of  the  object  is 
constant  regardless  of  the  width  of  the  lines.  Of 
course  in  making  the  photometric  balance  the  un- 
certainties of  color  photometry  are  present,  but  these 
are  not  of  much  importance  in  this  investigation, 
because  visual  acuity  changes  very  slowly  with 
change  in  brightness  of  the  object  at  the  illumination 
used;  therefore,  a  large  error  in  the  photometric  meas- 
urements would  cause  but  a  slight  error  in  the  visual 
acuity  measurements.  The  brightness  of  the  photo- 
metric field  as  seen  by  the  eye  through  the  artificial 
pupil  was  equivalent  to  the  brightness  of  a  white 
surface  illuminated  to  an  intensity  of  4.2  foot  candles. 
After  the  photometric  balance  was  made  by  varying 
the  current  through  the  large  lamp  illuminating  the 
test  object,  the  lamp,  j,  was  extinguished  and  a  series 
of  acuity  settings  were  made  by  varying  the  size  of 
the  lines.  The  results  obtained  are  shown  in  Fig. 
69.  Curves  a,  c,  represent  extreme  series  made  by 
the  author  showing  the  fluctuation  in  the  ability  of 
the  eye  to  distinguish  fine  details,  and  b  is  the  mean 
curve  of  a  great  many  observations.  Curves  d  and 
e  represent  single  series  of  observations  (ten  read- 
ings at  each  point)  made  by  two  other  observers. 


COLOR  AND  VISION 


135 


In  every  case  the  observer  was  permitted  to  focus 
the  instrument.  These  data  indicate  an  advantage 
in  the  defining  power  of  monochromatic  yellow  light 
over  other  monochromatic  lights  of  equal  brightness. 
In  order  to  extend  the  observations  into  the  violet 
end  of  the  spectrum,  the  test  object  was  illuminated 
by  means  of  a  mercury  arc.  The  mean  results  for 
each  of  two  observers  are  shown  in  Fig.  70,  for  three 
mercury  lines.  Curve  F  was  combined  with  curve 
b  in  Fig.  69  (obtained  by  the  same  observer)  which 


040  0.44  045  0.52  0.56  0.60  0.64  0.66 

Fig.  69.  —  Visual  acuity  in  monochromatic  lights  of  equal  brightness. 

extended  the  latter  as  indicated.  This  investiga- 
tion indicates  that  monochromatic  lights  differ  in 
their  defining  power  and  that  yellow  monochromatic 
light  is  superior  to  others  in  this  respect.  It  was 
also  found  that  for  a  given  change  in  brightness  of 
the  test  object  the  change  in  visual  acuity  was  least 
for  yellow  monochromatic  light  than  for  light  of  any 
other  spectral  hue. 

A  striking  experiment  illustrating  the  effect  of 
spectral  character  of  light  on  visual  acuity  is  given 
below.  The  test-object  was  viewed  through  an  ethyl 
violet  screen  (purple  under  the  illumination  from  a 
tungsten  lamp)  and  visual  acuity  settings  were  made* 
After  obtaining  the  mean  of  a  series  of  observations 


136 


COLOR  AND  ITS  APPLICATIONS 


a  yellow  screen  was  also  placed  before  the  eye.  This 
screen  absorbed  the  blue  and  violet  rays  transmitted 
by  the  purple  screen,  thus  reducing  the  illumination 
at  least  50  per  cent.  Notwithstanding  this  reduction 
in  illumination  visual  acuity  noticeably  increased. 
In  place  of  the  yellow  screen  was  now  substituted  a 
blue  screen  which  absorbed  the  red  rays  transmitted 


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Fig.  70.' — Visual  acuity  in  the  mercury  spectrum,  the  lines  being  reduced  to 

equal  brightness. 

by  the  purple  screen,  the  resulting  light  being  blue. 
Again  visual  acuity  increased,  notwithstanding  the 
reduction  in  brightness.  This  experiment  strikingly 
demonstrates  the  influences  of  chromatic  aberration 
and  spectral  character  of  light  on  the  ability  to  dis- 
tinguish fine  detail. 

It  is  interesting  to  note  some  results  on  the  legi- 
bility of  colored  advertisements.  Le  Courrier  du 
Livre  23  reported  the  legibility  of  various  combinations 


COLOR  AND  VISION  137 

for  reading  at  a  considerable  distance,  the  most  leg- 
ible print  being  black  on  a  yellow  background.  The 
order  of  merit  was  found  to  be  as  follows: 

1.  Black  on  yellow  8.  White  on  red 

2.  Green  on  white  9.  White  on  green 

3.  Red  on  white  10.  White  on  black 

4.  Blue  on  white  11.  Red  on  yellow 

5.  White  on  blue  12.  Green  on  red 

6.  Black  on  white  13.  Red  on  green 

7.  Yellow  on  black 

It  is  noteworthy  that  in  this  list  the  customary 
black-on-white  combination  is  sixth  on  the  list. 
These  results  are  interesting,  although  perhaps  not 
final,  owing  to  the  many  variables  that  enter  such  a 
problem. 

38.  Growth  and  Decay  of  Color  Sensations.  — 
Many  investigators  have  studied  the  problem  of  the 
effect  of  time  of  exposure  and  intensity  of  the  stim- 
ulus on  the  growth  and  decay  of  luminous  sensations. 
It  has  been  noted  (#  14)  that  colors  are  seen  on  rotat- 
ing, at  a  proper  speed,  a  disk  composed  of  black  and 
white  sectors.  It  appears  that  this  is  due,  in  part 
at  least,  to  the  difference  in  the  rate  of  growth  and 
decay  of  the  various  color  sensations  excited  by 
white  light.  Of  the  work  in  this  field,  that  of  Broca 
and  Sulzer24  is  especially  comprehensive.  They  com- 
pare the  brightness  of  a  white  screen  illuminated  by 
a  light  of  short  duration  with  that  due  to  a  standard 
steady  light.  Some  of  their  results  which  are  plotted 
in  Fig.  71  show  that,  excepting  for  lights  of  low  inten- 
sity, the  luminous  sensation  *  overshoots'  its  final 
value;  that  is,  the  maximum  luminous  sensation  is 
passed  a  comparatively  short  time  after  the  begin- 
ning of  the  exposure  and  that  the  luminous  sensa- 
tion reaches  a  steady  value  less  than  the  maximum 


138 


COLOR  AND   ITS  APPLICATIONS 


only  after  the  elapse  of  an  appreciable  fraction  of  a 
second  (depending  more  or  less  upon  the  intensity). 
The  numbers  on  the  curves  indicate  the  final  steady 
value  of  the  various  stimuli.  Their  data  obtained 
with  colored  light,  plotted  in  Fig.  72,  indicates  that 
under  the  stimulation  of  blue  rays  the  luminous 


OJ5          0.20 
SECONDS 

Fig.  71.  —  The  growth  and  decay  curves  for  white  light  sensation.     (Broca 

and  Sulzer.) 

sensation  overshoots  very  much  more  than  in  the 
case  of  red  or  green  light,  the  latter  showing  the 
least  overshooting. 

In  studying  the  growth  and  decay  of  color  sensa- 
tions in  connection  with  the  flicker  photometer  25  some 
data  of  interest  here  were  obtained.  Red  and  blue- 
green  lights,  practically  complementary,  were  matched 
by  the  ordinary  direct  comparison  method  of  photom- 


COLOR   AND    VISION 


139 


etry.  These  were  then  separately  flickered  against 
darkness  by  means  of  a  rotating  disk  with  equal  open 
and  closed  sectors.  The  maximum  brightness  of 
the  flickering  light  was  compared  with  a  steady 
brightness  of  the  same  color  for  a  large  range  of 
flicker  frequencies.  The  data  is  shown  in  Fig.  73, 


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Fig.  72. — The  growth  and  decay  curves  of  color  sensations. 

the  initials  R  and  G  representing  the  red  and  blue- 
green  lights  and  the  subscripts,  high  and  lower  in- 
tensities. The  intensities  used  were  those  ordinarily 
considered  satisfactory  in  photometry  as  is  indicated 
by  the  frequency  in  cycles  per  second  required  to 
cause  flicker  to  disappear.  It  will  be  noted  that  the 
colored  lights  were  alternated  against  darkness,  the 
steady  values  of  the  colored  lights  (sectors  open)  as 


140 


COLOR  AND   ITS  APPLICATIONS 


determined  by  the  direct  comparison  method  being 
represented  by  unity  on  the  relative  brightness  scale. 
The  flicker  of  GL,  /?L,  GH,  and  #H  completely  dis- 
appeared at  frequencies  corresponding  respectively  to 
A,  B,  C,  and  D. 

Next  red  and  blue-green  brightnesses  equivalent 
to  the  foregoing  were  placed  so  that  on  one  side  of 


RELATIVE  MAXIMUM  BRIGHTNESS 

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FLICKER  FREQUENCY  (CYCLES  PER  SECOND) 

Fig.  73. —  Showing  the  maxima  attained  by  flickering  lights  at  various 
frequencies. 

the  photometer  field  a  red  light  flickered  on  a  steady 
blue-green  field  and  vice  versa  on  the  other  side. 
This  was  done  by  means  of  identical  sectored  disks 
(180°  opening)  placed  one  on  each  side  of  the 
photometer.  On  one  side  a  disk  intercepted  the 
blue-green  light  and  on  the  other  the  red  light  was 
intercepted.  On  increasing  the  speed  of  rotation  of 
the  disks  (which  were  fastened  to  the  same  shaft)  the 
side  on  which  blue-green  light  flickered  upon  a  steady 
red  field  became  quiescent  long  before  the  flicker 


COLOR   AND   VISION 


141 


disappeared  on  the  other  side.  At  all  times  when 
flicker  was  visible  the  side  upon  which  red  flickered 
on  a  steady  blue-green  field  appeared  to  attain  higher 
maximum  values  of  brightness  and  to  be  more  agi- 
tated. The  brightnesses  on  either  side  were  later 


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FLICKER   FREQUENCY 

Fig.  74.  —  Showing  the  maxima  of  sensations  produced  by  flickering  red  light 
on  a  steady  green  field  (R),  and  vice  versa  (G)^ 

measured  separately  against  a  steady  white  light 
(there  being  little  color  difference  excepting  at  low 
speeds)  throughout  a  wide  range  of  frequencies. 
These  results  are  shown  in  Fig.  74,  R  and  G  indi- 
cating that  red  and  blue-green  were  respectively  the 
flickering  components.  The  steady  value  reached  at 


142  COLOR  AND   ITS  APPLICATIONS 

a  high  frequency  is  0.75,  unity  being  taken  as  the 
steady  value  at  zero  speed  with  the  sectors  open. 
The  latter  intercept  only  one  of  the  two  components 
which  make  up  the  brightness  on  either  side;  there- 
fore, the  sectors  being  of  50%  transmission,  the 
final  value  at  a  high  frequency  of  alternation  is  0.75 
of  the  original  steady  value  with  sectors  open.  Of 
course  these  experiments  involve  the  measurement 
of  the  brightness  of  surfaces  differing  in  color,  but 
it  is  this  problem  that  was  involved  in  the  study.  All 
steady  brightnesses  were  chosen  equal  as  measured 
by  an  ordinary  direct-comparison  photometer.  While 
these  effects  of  different  rates  of  growth  and  decay 
of  color  sensations  are  operative  when  there  is  an 
apparent  flicker,  evidence  points  to  the  disappearance 
of  such  influence  upon  the  brightness  of  a  mixture 
of  colored  light  by  alternately  presenting  the  colored 
stimuli  when  the  rate  of  alternation  is  so  high  that 
flicker  has  disappeared.  For  instance  the  foregoing 
red  and  blue-green  lights  were  mixed  by  alternating 
them  by  means  of  a  sectored  disk  (50%  opening) 
and  also  by  directly  superposing  the  steady  lights. 
The  former  mixture  was  found  to  be  just  one-half 
as  bright  as  the  latter,  within  the  slight  possible  errors 
of  the  experiment.  There  was  no  color  difference 
present  in  this  experiment  so  the  photometric  data 
is  correct  to  within  one  per  cent.  Other  evidence 
of  the  same  kind  was  obtained  by  comparing  two 
yellow  lights  of  the  same  hue,  but  differing  in  spectral 
character,  by  means  of  both  the  flicker  and  direct 
comparison  methods  of  photometry.  Identical  results 
were  obtained  by  the  two  methods.  These  results 
were  also  confirmed  by  comparing  tungsten  light 
by  the  two  methods  with  a  light  of  the  same  hue 
consisting  of  red  and  blue-green  lights.  (See  #  55.) 


COLOR   AND   VISION  143 

Talbot  26  long  ago  expounded  the  law  that  a 
sectored  disk  rotating  at  high  speed  transmitted 
light  in  direct  proportion  to  the  angular  openings  of 
the  sectors.  This  law  has  been  stated  by  Helmholtz27 
as  follows:  'If  any  part  of  the  retina  is  excited  with 
intermittent  light,  recurring  periodically  and  regularly 
in  the  same  way,  and  if  the  period  is  sufficiently 
short,  a  continuous  impression  will  result  which  is  the 
same  as  that  which  would  result  if  the  total  light  re- 
ceived during  each  period  were  uniformly  distributed 
throughout  the  whole  period."  Plateau,28  Kleiner,29 
Weideman  and  Messerschmidt,30  Ferry,31  Lummer  and 
Brodhun,32  Aubert,33  Hyde,34  and  others  have  inves- 
tigated the  problem,  and  have  generally  agreed  that 
the  law  holds  for  white  light.  Fick  concluded  that 
it  holds  only  at  moderate  intensities  and  Ferry  veri- 
fied the  law  for  white  light  but  found  discrepancies 
when  one  side  of  the  photometer  field  was  bluish  as 
compared  to  the  other  side.  Hyde,  after  a  thorough 
investigation  of  the  problem,  concluded  that  the  law 
holds  within  the  accuracy  of  the  work  (about  0.3%) 
for  the  range  of  sectors  used  by  him,  namely  from 
288°  to  10°  in  opening.  He  further  concluded  that 
the  law  held  for  red,  green,  and  blue  lights  within 
the  accuracy  of  precision  photometric  apparatus,  and 
found  that  when  a  color  difference  existed  on  the 
two  sides  of  the  photometer  field  no  appreciable 
deviation  from  the  law  was  observed.  The  author  has 
had  many  opportunities  to  test  the  law  for  colored 
lights  and  found  no  deviations  within  the  accuracy 
of  the  experimental  work,  which  was  usually  well 
within  one  per  cent.  The  sectored  disk,  there- 
fore, affords  a  means  of  altering  the  intensity  of 
colored  light  in  definitely  measurable  amounts. 

Lights   of  very   short   duration   are   perceptible   if 


144  COLOR  AND   ITS  APPLICATIONS 

intense  enough.  For  instance,  a  lightning  flash  as 
short  as  one-millionth  of  a  second  is  visible  and  by 
rotating  mirrors  flashes  of  light  as  short  as  one  eight- 
millionth  of  a  second  have  been  perceived.  Blondel 
and  Rey  35  studied  the  perception  of  lights  of  short 
duration  at  their  range  limits.  Bloch  36  had  pre- 
viously contended  that  the  excitation  necessary  for 
the  production  of  the  minimum  sensation  was  per- 
ceptibly constant  and  proportional  to  the  product  of 
the  brightness  and  the  duration.  Charpentier 37  veri- 
fied the  law  within  certain  limits.  Blondel  and  Rey 
conclude  that  Bloch's  law  can  be  applied  only  to 
intense  lights  of  very  short  duration.  After  a  very 
extended  investigation  they  deduce  a  simple  law, 
(B  —  Bo)t=aB0,  where  B0  is  the  minimum  per- 
ceptible brightness  of  the  field,  t  the  duration  of  the 
stimulus  in  seconds,  and  a  is  a  constant  of  time  equal 
to  0.21  second.  They  show  by  simple  integration 
one  can  deduce  from  the  law  of  the  flashes  which 
are  not  uniform,  their  range  and  the  intensity  of  the 
equivalent  constant  light  from  the  point  of  view  of 
range, 


where  7h  represents  the  photometric  intensities  of 
the  luminous  points  measured  in  a  horizontal  section 
of  the  beam  and  referred  to  unit  distance.  They 
conclude  by  taking  into  consideration  the  curves  of 
sensation  of  Broca  and  Sulzer  24  'that  the  maximum 
utilization  of  a  source  of  light  must  demand  short 
flashes  without  its  being  necessary  to  take  any 
notice  of  an  inferior  limit  of  the  period  of  the  signals, 
except  in  the  case  of  telegraphic  signals.  It  more- 


COLOR   AND   VISION 


145 


over  suffices  that  the  period  of  the  flash,  £2-£i,  should 
become  a  negligible  quantity  in  the  presence  of  the 
constant  a,  in  order  that  a  maximum  efficiency  may 
be  assured.' 

On  alternating  a  given  brightness  with  darkness 
by  means  of  a  sectored  disk  with  50%  openings,  a 
violent  flicker  is  evident  at  low  speeds;  however, 
there  is  a  certain  minimum  frequency,  called  the 
critical  frequency,  at  which  the  flicker  just  disappears. 
The  critical  frequency  depends  upon  the  intensity  of 
illumination  or  brightness  of  the  observed  field  and 
increases  with  the  brightness.  Porter 38  has  found 


LOGARITHM  OF  BRIGHTNESS 

Fig.  75.  —  Showing  the  relation  between  brightness  and  critical  frequency  for 

colored  stimuli. 

that  the  relationship,  /  =  a  log  7+6,  holds  for  white 
light  where  /  is  the  critical  frequency,  /,  the  illumi- 
nation, and  a  and  b  are  constants;  that  is,  there  is 
a  straight  line  relation  between  the  critical  frequency 
and  the  logarithm  of  illumination.  The  constant,  a, 
has  two  values,  one  for  brightnesses  above  those 
resulting  from  illuminations  on  a  white  surface  greater 
than  about  0.25  meter  candles  and  one  below.  It 
is  thought  by  adherents  to  the  von  Kries  'duplicity 


146  COLOR  AND   ITS  APPLICATIONS 

theory'  (#48)  that  this  point  of  abrupt  change  in  slope 
corresponds  to  the  change  from  cone  to  rod  vision. 
Haycraft39  has  studied  the  critical  frequency  for 
spectral  lights,  but  the  results  are  complicated,  be- 
cause the  intensity  of  the  various  rays  was  not  con- 
stant throughout  the  spectrum.  Ives40  studied  the 
relation  between  critical  frequency  and  brightness  for 
various  spectral  rays  and  obtained  results  which  he 
expresses  diagrammatically  as  shown  in  Fig.  75,  the 
logarithm  of  brightness  being  plotted  against  critical 
frequency.  It  is  noted  that  the  'red'  curve  shows 
no  change  in  direction  at  low  intensities.  The  blue 
curve  changes  from  a  diagonal  to  a  horizontal  straight 
line;  that  is,  at  low  illuminations  the  critical  fre- 
quency becomes  constant  for  blue  light  of  various 
feeble  intensities.  Intermediate  curves  represent  spec- 
tral colors  between  red  and  blue.  It  is  significant 
to  note  that  the  slopes  of  the  curves  are  different  for 
the  higher  illuminations,  the  'blue'  slope  being 
steeper  than  the  'red'  slope,  which  indicates  that 
the  Purkinje  phenomenon  is  operative. 

The  author  25  has  shown  that  the  critical  frequency 
depends  upon  the  wave  form  of  the  stimulus  or  the 
contour  of  flicker.  Some  of  the  data  for  white  light 
is  shown  in  Fig.  76.  In  cases  a,  6,  c,  the  maximum, 
minimum,  and  mean  cyclic  illumination  were  re- 
spectively the  same.  A  difference  in  critical  fre- 
quency was  obtained  throughout  a  wide  range  of 
illumination,  the  critical  frequency  being  higher  the 
greater  the  period  of  darkness  in  a  given  cycle. 
This  also  appeared  to  hold  for  colored  lights,  but  no 
extensive  study  has  yet  been  made. 

39.  Signaling.  —  The  chief  requisite  of  a  colored 
light  for  signaling  purposes  is  high  intensity,  because 
its  range  depends  largely  upon  this  factor..  This 


COLOR   AND   VISION 


147 


precludes  the  use  of  very  pure  colors  owing  to  low 
intensities  obtainable  in  practise,  and  for  this  reason 
signal  glasses  are  a  compromise  between  saturation 
of  color  and  transparency.  As  is  seen  by  the  redness 
of  the  setting  sun,  red  rays  are  less  absorbed  by 
smoke  and  dust  in  the  atmosphere  than  the  blue  rays, 
therefore,  a  red  signal  should  have  a  greater  range 
than  a  blue  signal  through  a  smoke  and  dust  laden 


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Fig.  76.  —  Effect  of  contour  of  flicker  on  critical  or  vanishing-flicker  frequency. 

atmosphere.  Churchill41  quotes  results  obtained  by  the 
Geheimrat  Koerter  of  the  German  Lighthouse  Board  on 
the  selective  absorption  of  artificial  fog.  The  results 
indicated  that  red  rays  were  absorbed  to  a  greater 
degree  (about  20%  more)  than  blue  rays.  These 
results  indicate  that  the  selective  absorption  of  clouds 
plays  no  part  in  the  shifting  of  the  color  of  the  setting 
sun  toward  red.  On  the  other  hand  the  German 
Government  in  about  1900  found  that  an  installa- 
tion of  arc  lamps  in  a  lighthouse  on  the  island  of 


148  COLOR  AND   ITS  APPLICATIONS 

Heligoland  had  a  lesser  range  in  a  fog  than  a  kero- 
sene light  of  only  one-hundredth  the  candle-power. 
The  former  contains  a  predominance  of  blue  rays  in 
comparison  with  the  latter,  which  from  the  foregoing 
tests  in  artificial  fog  apparently  suffered  less  absorp- 
tion. 

The  German  Lighthouse  Board  of  Hamburg  in 
1894  carried  out  an  extensive  series  of  tests  on  the 
range  of  signal  lights  with  the  following  results  as 
presented  by  Churchill,  where  R  represents  the  range 
in  miles  and  /  the  candle-power. 

For  white  light  in  clear  weather  R  =  1.53\/I 
For  white  light  in  rainy  weather  R  =  1.09  VI 
For  green  light  in  clear  weather  R  =  1.6  \/ 1 

M.  Busstyn  42  estimates  the  range  of  red  light  as 
R  =  1.5 Vl.  The  spectral  character  of  the  illuminant 
of  course  has  an  important  influence  on  the  color 
of  the  signal  glass. 

It  is  interesting  to  note  that  on  a  certain  modern 
battleship  a  lighting  system  of  blue  lamps  has  been 
installed  for  use  at  night  when  in  action.  The  reason 
given  for  installing  blue  lights  is  that  they  are  in- 
visible to  the  enemy.  No  information  was  obtain- 
able as  to  whether  the  short  range  is  due  to  the 
faintness  of  the  blue  lights  or  to  a  supposed  lower 
range  for  blue  than  for  yellow  light  of  equal  intensity. 

The  Railway  Signal  Association  (1908)  after  exten- 
sive tests  arrived  at  the  conclusions  expressed  in 
Table  XIII  for  the  effective  range  of  the  principal 
signal  colors  under  average  weather  conditions.  The 
colored  glasses  are  assumed  to  be  used  with  the 
customary  semaphore  lamp  and  lens. 

Paterson  and  Dudding  43  have  performed  some 
interesting  experiments  on  the  visibility  of  point 


COLOR   AND   VISION 


149 


TABLE    XIII 


Color 

Effective  range 
(miles) 

Approx.  transmission 
coef.  of  glass  in  service 

Red                         

3  to  3.5 

0.20 

Yellow 

1  to  1.5 

.35 

Green  

2.5  to  3.0 

.17 

Blue  
Purple 

0.5  to  0.75 
0.5  to  0.75 

.03 
.03 

Lunar-white 

2  to  2.5 

.15 

sources  made  by  placing  plates  containing  minute 
apertures  before  a  wax  flame.  While  most  of  their 
work  was  done  indoors  at  distances  as  great  as  550 
feet,  an  experiment  on  the  visibility  of  the  light  from 
tungsten  and  carbon  incandescent  lamps  over  ranges 
which  extended  more  than  a  mile  showed  no  differ- 
ence in  the  carrying  power  of  these  lights  on  a  clear 
night.  They  established  the  theorem  that  the  visi- 
bility of  a  point  source  is  proportional  to  the  candle- 
power  of  the  source  and  to  the  inverse  square  of 
the  distance.  They  also  found  that  the  visibility 
is  independent  of  the  intrinsic  brightness  for  sources 
subtending  less  than  two  minutes  of  arc.  In  this 
connection  it  might  be  noted  that  they  assumed  a 
point  source  to  be  one  whose  linear  dimensions  sub- 
tend an  angle  at  the  eye  less  than  the  resolving 
power  of  the  eye,  i.  e.  about  30  seconds  of  arc  for 
a  mean  wave-length  of  0.5/x  and  pupillary  aperture  of 
4.5  mm.  They  found  that  the  visibilities  of  red  and 
green  lights  in  clear  air  were  closely  proportional  to 
the  inverse  square  of  the  distance.  On  slightly 
illuminating  the  field  surrounding  the  point  source 
there  was  a  loss  in  visibility  of  about  10%  for  a  red 
light,  15%  for  a  white  light,  and  18%  for  a  green 
light,  all  being  of  equal  candle-power.  Their  method 


150 


COLOR  AND   ITS  APPLICATIONS 


of  determining  the  candle-power  of  the  red  and  green 
lights  in  the  latter  experiment  was  to  determine  the 
visibility  of  the  unknown  in  terms  of  the  visibility 
of  a  white  light  of  known  candle-power  assum- 
ing the  visibility  porportional  to  the  candle-power 
and  the  inverse  square  of  the  distance.  Their  unit 
of  visibility  was  equivalent  to  the  visibility  of  a  point 
source  of  one  candle-power  at  1000  meters  distance, 
and  they  state  that  the  lowest  visibility  considered 
desirable  was  0.12  of  this  unit.  Their  results  for 
white  light  agree  fairly  well  with  those  obtained  by  the 
Deutsche  Seewarte  of  1894,  as  is  shown  in  Table  XIV. 

TABLE   XIV 


Range 

(Deutsche  Seewarte) 
Candle  power  re- 
quired in  clear 
weather 

(Paterson  &  Dudding) 
Computed  from 
results  of 
experiment 

1  sea-mile    (1855  meters)  

0.47 

0.41 

2  sea-miles  (3710  meters)  
5  sea-miles  (9275  meters)  

1.9 
11.8 

1.6 
10.0 

By  using  artificial  pupils  they  found  little  evidence 
of  any  influence  of  "the  spherical  aberration  of  the 
eye  on  the  visibility  of  point  sources.  They  showed 
by  using  positive  lenses  that  a  green  light  equivalent 
to  a  point  source  was  greatly  dimmed  relatively  to  a 
red  light  of  similar  dimension,  which  they  attribute 
to  the  chromatic  aberration  of  the  eye.  They  con- 
clude that  unless  an  observer  has  sufficient  accom- 
modation available  to  focus  properly  a  green  light  at 
infinity  the  latter  will  appear  dimmed  in  proportion 
to  the  amount  this  image  is  out  of  focus.  This  is 
not  so  likely  to  occur  with  red  light,  because  images 
of  this  color  do  not  require  as  much  accommodation 


COLOR  AND   VISION  151 

in  order  to  focus  them  on  the  retina.  A  purple 
roundel  at  some  distance  sometimes  appears  red  in 
the  center  with  blue  diverging  from  it,  which  is  attrib- 
uted to  chromatic  aberration  of  the  signal  lens. 

It  is  hardly  necessary  to  state  the  importance  of 
tests  for  color-blindness  of  eyes  engaged  in  dis- 
criminating colored  signals.  Numerous  test  methods 
have  been  devised,  but  one  that  has  been  used  very 
much  is  the  Holmgren  test,  conducted  with  colored 
skeins  of  wool. 

In  the  choice  of  signal  colors  the  four  most  dis- 
tinctive are  red,  yellow,  green,  and  blue.  To  these 
can  be  added  white  (clear)  and  purple.  The 
blue  and  purple  owing  to  their  low  intensity  are 
suitable  only  for  short-range  signals.  Results  by 
Churchill  on  the  reaction  times,  or  the  intervals  re- 
quired to  distinguish  and  name  signals  of  different 
colors,  were  in  general  in  the  following  order.  Red 
was  recognized  and  named  in  the  shortest  time, 
green  ranked  next,  then  yellow,  and  lastly  white, 
which  required  the  longest  interval  for  recognition. 
The  relative  length  of  the  time  interval  varied  with 
different  subjects,  but  the  order  given  was  found  to 
be  generally  the  case.  Of  course  the  spectral  char- 
acter of  the  illuminant  has  an  important  influence  on 
the  color  of  the  signal  glass. 

40.  Other  Uses  for  Colored  Glasses.  —  It  is  well 
known  that  dust  and  smoke  (and  very  likely  fog) 
scatter  the  visible  rays  of  short  wave-length  more 
than  those  of  longer  wave-length.  For  this  reason 
it  is  contended  by  some  that,  if  the  blue  and  violet 
rays  are  subtracted  from  white  light,  the  remaining 
light  (yellow  in  color)  will  enable  an  observer  to  see 
further  than  the  total  light.  It  is  of  interest  to  inquire 
further  into  the  matter.  In  the  case  of  the  search- 


152  COLOR  AND  ITS  APPLICATIONS 

light,  if  the  operator  wishes  to  see  a  distant  object 
in  a  fog  he  is  required  to  look  through  an  illuminated 
veil  caused  by  scattered  light,  which  results  in  de- 
creasing the  ability  to  distinguish  the  object.  If 
it  is  true  that  the  blue  and  violet  rays  are  scattered 
more  by  the  fog  particles,  the  luminous  veil  would 
become  more  annoying  for  lights  containing  relatively 
greater  amounts  of  blue  rays.  Rough  quantitative 
tests  by  the  author,  employing  auto  lamps  with  para- 
bolic reflectors,  indicated  that  with  yellow  light,  which 
was  less  intense  than  the  total  light  by  the  amount 
of  light  absorbed  by  the  yellow  screen,  objects  in  a 
fog  could  be  seen  more  clearly  than  with  the  total 
light.  There  is  another  view-point,  namely  that  of 
the  person  who  desires  to  distinguish  a  light  signal 
at  a  considerable  distance.  Here  again  an  illumi- 
nated veil  relatively  near  the  light  source  if  visible 
is  likely  to  decrease  the  visibility  of  the  signal  light. 
This  point,  however,  requires  investigation. 

Based  on  the  foregoing  principle  many  patents 
have  been  obtained  for  methods  of  eliminating  the 
violet  rays.  Colored  glasses,  gold-plated  reflectors, 
fluorescent  glass  reflectors,  etc.,  have  been  employed, 
but  all  for  the  same  object.  A  noteworthy  problem 
of  projection  arises  with  the  carbon  arc  search-light. 
In  order  to  obtain  a  beam  of  parallel  light  by  means 
of  silvered  reflectors  the  area  that  emits  light  must 
be  small  and  be  located  at  the  focus  of  a  parabolic  re- 
flector. 'With  high-amperage  arcs  an  appreciable 
portion  of  the  light  is  emitted  by  the  arc  flame  of 
relatively  large  area  as  compared  with  the  crater 
of  the  positive  carbon  which  is  located  at  the  focus  of 
the  parabolic  reflector.  The  light  from  the  arc  flame 
has  a  decided  violet  tinge  as  compared  with  the  light 
from  the  crater,  and  furthermore,  being  out  of  the  focus 


COLOR   AND   VISION  153 

of  the  parabolic  reflector,  its  light  is  not  *  paralleled,' 
but  escapes  in  a  cone  of  relatively  large  angle.  By 
using  a  yellowish  glass  in  the  aperture  of  the  search- 
light, this  light  of  a  bluish  tinge  is  greatly  reduced  in 
comparison  with  the  reduction  of  the  yellower  light 
from  the  crater,  thus  decreasing  the  possible  annoy- 
ance due  to  the  'luminous  veil.'  The  same  result 
would  be  obtained  if  the  'look-out'  wore  yellow 
glasses.  In  the  case  of  a  very  powerful  searchlight 
such  a  glass  in  the  aperture  probably  would  be  broken 
by  the  rise  in  temperature  due  to  its  absorption  of 
radiant  energy.  If  it  were  inconvenient  for  the  look- 
outs to  wear  the  yellow  glasses  before  their  eyes 
there  would  be  some  virtue  in  the  gold-plated  re- 
flector which  would  reduce  the  amount  of  blue  and 
violet  light  in  the  reflected  light.  However,  in  all 
such  cases  there  is  a  cone  of  light  which  escapes 
directly  without  being  altered  by  selective  reflection* 
In  the  application  of  the  foregoing  principle  to  auto 
headlights  or  to  any  projectors  employing  electric 
incandescent  lamps,  any  possible  objectionable  effect 
of  the  excessive  scattering  of  violet  and  blue  rays 
can  be  overcome  by  incorporating  the  yellow  glass 
directly  in  the  lamp  bulb,  by  applying  a  yellow  color- 
ing to  the  exterior  of  the  bulb,  by  inserting  a  yellow 
glass  in  the  aperture  of  the  reflector,  or  by  wearing 
yellowish  glasses  before  the  eyes.  An  interesting 
case  is  found  in  a  fluorescent  glass  reflector  (silvered 
on  the  back  surface)  which  absorbs  most  of  the 
violet  and  blue  rays.  One  of  the  claims  advanced 
for  this  reflector  is  that  it  utilizes  the  ultra-violet, 
violet,  and  blue  rays  of  the  incandescent  lamp  by 
taking  advantage  of  the  fluorescent  property  of  ura- 
nium glass  which  converts  these  rays  into  yellow- 
green  light;  however,  these  rays  constitute  a  very 


154  COLOR  AND   ITS  APPLICATIONS 

small  proportion  of  the  total  visible  rays  in  the  light 
from  electric  incandescent  lamps  ordinarily  used  for 
such  purposes.  Furthermore,  the  fluorescent  yellow- 
green  light  produced  by  these  rays  is  not  '  directed ' 
by  the  parabolic  reflector,  because  this  light  is  emitted 
in  all  directions.  On  looking  at  such  a  reflector  it 
appears  of  a  yellowish  green  tint,  but  close  examina- 
tion shows  that  the  yellow-green  fluorescent  light 
is  emitted  by  the  glass  in  all  directions.  Therefore, 
no  practical  gain  in  intensity  of  the  directed  beam 
results  from  the  conversion  of  the  ultra-violet,  violet, 
and  blue  rays  into  yellow-green  light,  because  the 
latter  is  diffusely  emitted  and  the  effect  of  such  a 
fluorescent  glass  amounts  to  little  more  than  elimi- 
nating most  of  the  violet  and  blue  rays  from  the 
radiation  that  is  intercepted  by  the  reflector.  In 
this  case  there  is  also  a  cone  of  unaltered  light,  equal 
in  solid  angle  to  that  subtended  by  the  aperture  of  the 
reflector,  which  escapes  *  undirected.*  This  scheme 
appears  to  have  little  value,  inasmuch  as  a  yellow 
glass  in  the  aperture  of  the  reflector  would  accomplish 
the  purpose  in  a  more  satisfactory  and  simple  manner. 
Amber,  yellow,  and  greenish  yellow  glasses  have 
been  used  successfully  for  eliminating  glare  from  the 
blue  sky.  Riflemen  have  found  such  glasses  of 
extreme  value  in  range  shooting  and  a  number  of 
sportsman's  glasses  are  available  in  the  market.  In 
the  case  of  amber  or  greenish  yellow  glasses  the 
improved  condition  of  seeing  is  perhaps  largely  due 
to  the  reduction  of  glare  from  the  blue  sky,  but  also 
in  part  to  an  increased  defining  power  due  to  the 
elimination  of  blue  and  violet  rays  and  a  relative 
reduction  of  the  extreme  red  rays  (#37).  An  illus- 
tration of  the  effect  of  greenish-yellow  glasses  44  in 
increasing  the  ease  of  distinguishing  detail  is  shown 


COLOR   AND   VISION 


155 


in  Fig.  77.  An  acuity  object  15  (#37)  was  set  up  in 
the  shade  of  a  building  on  a  clear  day  and  light 
reached  the  object  from  at  least  one-half  of  the  open 
sky.  No  direct  light  from  the  sun  reached  the  eye, 
test  object,  or  immediate  surroundings.  The  author 
who  made  the  observations  wore  no  visor  to  shield 
the  eyes.  Only  a  slight  sensation  of  discomfort  was 
apparent  before  beginning  the  test;  however,  as  soon 
as  acuity  observations  were  begun  the  glare  became 
very  evident  and  rapidly  grew  painful.  Five  readings 


0  3  6  9  12  15  18 

TIMECMINUTES) 

Fig.  77. —  Effect  of  yellow-green  glasses  on  vision  under  a  bright  sky. 

were  made  first  through  clear  correcting  glasses  (rep- 
resented by  the  black  dots)  and  as  quickly  as  pos- 
sible the  clear  glasses  were  replaced  by  yellow-green 
glasses  of  about  50  per  cent  transmission  for  the 
total  light  and  five  acuity  readings  were  taken  (repre- 
sented by  crosses).  A  decided  decrease  in  discom- 
fort was  experienced  when  wearing  the  yellow-green 
glasses  and,  as  will  be  noted,  visual  acuity  is 
higher  in  this  case,  notwithstanding  the  decrease  in 
illumination  was  fully  50  per  cent.  These  glasses 
were  again  replaced  by  clear  glasses  and  five  acuity 
readings  were  made.  This  procedure  was  continued 
as  indicated  in  Fig.  77.  The  interval  of  time  required 


156  COLOR  AND   ITS  APPLICATIONS 

to  make  five  readings  including  the  change  of  glasses 
was  the  same  in  each  case  (being  three  minutes), 
but  the  actual  time  of  making  the  individual  readings 
was  not  noted;  therefore,  they  are  plotted  at  equal 
intervals.  While  the  above  conditions  are  rather 
complex  and  involve  problems  worthy  of  much  careful 
investigation,  the  experiment  answered  the  intended 
purpose  in  bringing  forth  several  points:  (1)  Glare 
conditions  are  not  always  apparent  when  the  eyes 
are  not  engaged  in  serious  work  such  as  reading  or 
distinguishing  fine  detail.  However,  bad  lighting 
conditions  are  readily  recognized  when  the  eyes  are 
called  upon  to  do  such  work.  (2)  There  is  a  rapid 
falling  off  of  visual  acuity  when  the  conditions  of 
glare  are  severe.  (3)  Such  a  harmless  appearing 
light  source  as  a  wide  expanse  of  sky  can  produce 
a  very  severe  condition  of  glare.  The  intrinsic 
brightness  is  very  low  as  compared  with  artificial 
sources,  but  the  quantity  of  light  is  high  and  the 
image  of  the  sky  is  spread  over  a  large  portion  of  the 
retina.  Its  annoyance  can  be  decreased  by  the  use 
of  colored  glasses,  which  absorb  much  of  the  blue 
light.  (4)  There  was  an  apparent  recuperation  of  the 
eye  during  the  periods  that  the  yellow-green  glasses 
were  worn.  (5)  Notwithstanding  the  effect  of  glare 
(when  clear  glasses  were  worn)  in  reducing  visual 
acuity  the  values  of  the  latter  when  the  colored 
glasses  were  worn  remained  considerably  higher. 
(6)  This  experiment  emphasizes  the  necessity  of 
prolonging  acuity  readings  over  a  considerable  period 
if  acuity  is  to  be  a  criterion  of  the  satisfactoriness 
of  illumination  conditions.  Some  of  the  increase  in 
visual  acuity  when  the  yellow-green  glasses  were 
being  worn  can  be  accounted  for  by  the  nearer  ap- 
proach to  monochromatism  of  the  light  that  passed 


COLOR   AND   VISION  157 

through  them.  However,  conditions  indicated  that 
the  advantage  was  due  very  largely  to  a  reduction  in 
the  glare  from  the  sky  because  the  glasses  absorbed 
much  of  the  blue  and  violet  light.  Other  interesting 
conclusions  can  be  drawn,  but  the  illustration  has 
already  fulfilled  its  object  in  bringing  forth  the  fact 
that  glare  conditions  are  very  complex  and  that  cog- 
nizance of  glare  often  depends  upon  the  character  of 
the  activities  in  which  the  eyes  are  engaged. 

Glasses  for  protecting  the  eyes  from  visible,  ultra- 
violet, or  invisible  rays  are  coming  into  prominence. 
In  considering  only  the  visible  rays,  colored  glasses 
may  be  combined  after  the  principle  of  the  subtractive 
method  of  mixing  colors  (#18,  Fig.  20,  Plate  II).  A 
superabundance  of  violet,  blue,  or  green  rays  can  be 
reduced  by  the  use  of  red  glass.  That  is,  a  colored 
glass  will  greatly  reduce  rays  roughly  complementary 
in  color.  Spectrophotometric  analyses,  affording  data 
such  as  are  shown  in  Fig.  12,  are  quite  necessary  for 
intelligently  combining  glasses  for  protecting  the 
eyes.  Spectrophotographic  analysis  is  a  convenient 
means  of  studying  the  transmission  characteristics 
of  glasses  in  the  ultra-violet  region  and  radiometric 
methods  are  applicable  to  the  infrared  region.  Ordi- 
nary glass  is  sufficiently  protective  against  moderate 
amounts  of  ultra-violet  energy.  In  ordinary  lens 
thicknesses  it  is  transparent  to  about  0.360/*,  from 
which  point  it  begins  to  absorb  ultra-violet  rays,  be- 
coming practically  opaque  to  rays  of  shorter  wave- 
length than  0.300/z.  Some  green,  yellow,  orange,  and 
red  glasses  are  totally  opaque  to  all  ultra-violet  rays, 
but  this  cannot  be  ascertained  by  a  mere  visual 
inspection.  However,  the  ultra-violet  transmission 
can  be  roughly  determined  by  means  of  a  quartz 
spectrograph  and  an  iron  arc  or  a  quartz  mercury  arc. 


158 


COLOR    AND    ITS    APPLICATIONS 


On  focusing  the  spectrum  of  the  radiation  emitted 
by  the  quartz  arc  on  a  fluorescent  material  such  as 
uranium  glass,  the  various  ultra-violet  lines  will  be 
seen  owing  to  their  production  of  fluorescence.  On 
inserting  a  specimen  of  glass  before  the  slit  of  the 
spectrograph  the  region  of  absorption  will  be  readily 
perceived  by  the  disappearance  or  decrease  in  bright- 
ness of  various  fluorescent  lines.  The  transmission 


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/ 

y 

/9 

"> 

-^ 

-^».. 

s 

—  — 

^ 

^    ™ 

/ 

-zr 

•^ 

x 

>.5U      UM         0.34        0.36        0.58         0.40        042        044        046        048 
JJ;.  WAVE  LENGTH 

0.5 

Fig.  78.  —  Ultra-violet  transmission  curves  of  various  glasses. 


of  glasses  in  the  ultra-violet  region  45  has  been  de- 
termined by  using  a  wide  slit,  the  spectrum  of  the 
quartz  mercury  arc,  and  a  combined  photographic 
and  photometric  method.  For  qualitative  analysis 
an  iron  arc  is  a  satisfactory  source  rich  in  ultra-violet 
rays.  Some  specimen  transmission  curves  for  various 
optical  glasses,  employed  for  protecting  the  eyes 
from  ultra-violet  energy,  are  shown  in  Fig.  78,  the 
ultra-violet  region  being  represented  to  the  left  of 
the  heavy  vertical  line  at  0.40/x.  It  is  interesting 


COLOR  AND   VISION  159 

to  note  the  difference  in  ultra-violet  absorption  of  the 
two  samples  of  '  smoke '  glass.  Of  course  the  absorp- 
tion depends  upon  the  thickness  of  the  specimen  and 
its  density  of  color.  All  the  glasses  excepting  three 
specimens,  3,  5,  and  6,  transmitted  50%  or  more  of  the 
total  light  from  a  tungsten  lamp.  Spectrophoto- 
graphic  analyses  of  various  glasses  are  shown  in  Fig. 
18. 

In  general  it  is  no  doubt  advisable  to  use  glasses 
as  free  from  color  as  possible  and  yet  providing 
protection  if  they  are  to  be  worn  for  long  periods. 
Yellow-green  glasses  when  otherwise  filling  the  re- 
quirements appear  to  distort  colors  (more  commonly 
encountered)  less  than  medium  amber.  A  striking 
instance  was  found  in  the  lap-welding  department 
of  a  steel  mill,  where  the  operators  judge  tempera- 
ture visually.  They  became  confused  when  wear- 
ing amber  glasses,  but  found  no  difficulty  in  using 
yellow-green  glasses.  This  brings  to  mind  the  fact 
that  through  a  yellow-green  glass  transmitting  only 
a  limited  region  of  the  spectrum  the  relation  of  bright- 
ness and  temperature  appears  practically  the  same 
as  to  the  unobstructed  eye  when  the  luminous  sub- 
stance radiates  light  approximately  the  same  as  a 
black  or  'gray'  body.  Years  ago  Crova  suggested 
a  method  of  photometry  involving  this  principle  (#  54). 
Schanz  and  Stockhausen,  Voege,  Crookes,  Parsons, 
and  others  have  studied  the  subject  of  protecting  the 
eye  from  harmful  rays.  Crookes  46  concludes  with 
his  associates  that  the  relatively  great  amounts  of 
infrared  energy  emitted  by  molten  glass  is  responsible 
for  glass-blowers'  cataract,  although  this  conclusion 
is  questioned  by  some.  He  has  made  an  exhaustive 
study  of  the  manufacture  of  glasses  for  eye-protection 
and  has  published  the  valuable  results. 


160  COLOR  AND   ITS  APPLICATIONS 

Colored  glasses  are  often  used  for  bringing  out 
certain  colored  portions  of  an  object  in  more  striking 
contrast  with  the  surroundings.  For  instance,  if  a 
black-line  drawing  be  made  on  blue-lined  coordinate 
paper  and  viewed  through  a  dense  blue  glass,  the 
blue  lines  practically  disappear.  If  the  drawing  be 
photographed  through  this  glass  the  coordinate  lines 
will  not  appear  on  the  negative.  In  the  same  manner 
if  blue  and  red  appear  upon  the  same  background, 
one  or  the  other  can  be  made  practically  to  disappear 
by  using  a  colored  screen  of  exactly  the  same  color. 
Of  course  the  degree  of  change  in  contrast  will  depend 
upon  the  purity  of  the  colors  and  the  care  exercised 
in  choosing  the  colored  screen. 

In  using  field  glasses  distant  vision  can  be  im- 
proved sometimes  by  the  use  of  a  light  yellow  screen 
which  eliminates  the  blue  haze  from  the  visual 
image.  In  this  connection  it  is  well  to  note  also  that 
blue  rays  are  normally  out  of  focus  at  the  retina. 
The  author  has  experimented  with  colored  screens 
for  use  with  field  glasses  for  detecting  colored  objects 
at  a  distance  by  altering  their  contrast  with  the  sur- 
roundings by  the  use  of  colored  screens.  For  in- 
stance, a  khaki  uniform  (yellow-orange  in  color)  can 
be  made  to  appear  either  lighter  or  darker  than  the 
green  foliage  surrounding  it  by  respectively  using  a 
yellow-orange  screen  or  one  of  a  complementary  hue. 
For  instance  if  the  ratio  of  the  brightness  of  a  piece 
of  khaki  cloth  to  that  of  a  certain  green  leaf  be 
taken  as  unity  under  daylight  illumination,  through 
an  ordinary  orange  filter  this  ratio  became  1.5  and 
through  a  blue-green  filter,  0.7.  With  care  the  con- 
trast can  be  made  practically  a  maximum.  In  the 
case  of  objects  more  striking  in  color  the  problem  is 
not  as  difficult.  Whether  or  not  the  reduction  of 


COLOR   AND   VISION  161 

brightness  more  than  offsets  the  advantage  of  in- 
creased contrast  in  distinguishing  distant  objects  can 
be  solved  by  actual  trial.  The  point  is  mentioned 
here  to  illustrate  the  possibilities  in  the  use  of  colored 
glasses  as  an  aid  to  vision. 

REFERENCES 

1.  Physiol.  Optik.   1896,   p.   140. 

2.  Sitz.  d.  Berliner  Akad.  1888,  p.  917. 

3.  Bui.  Bur.  Stds.  1907,  p.  59. 

4.  Lancet,  Oct.  2,  1909. 

5.  Proc.  Roy.  Soc.  A,  84,  p.  464. 

6.  Color  Scales. 

7.  Wein.  Sitz.  1906,  II  a,  115,  p.  1. 

8.  Bui.  Bur.  Stds.  6,  p.  89. 

9.  Bui.  Bur.  Stds.  9,  p.  59. 

10.  Trans.  I.  E.  S.  1914,  9,  p.  700. 

11.  Physiol.  d.  Netzhaut,  Breslau,  1865,  p.  138. 

12.  Amer.  Jour.  Psych.  1913,  24,  p.  171. 

13.  Elec.  World,  1911,  57,  p.  1163. 

14.  Elec.  World,  1911,  58,  p.  450. 

15.  Elec.  World,  1910,  55,  p.  939. 

16.  Elec.  World,  Dec.  6,  1913. 

17.  Lon.   Ilium.  Engr.  2,  p.  233. 

18.  Elec.  World,  Feb.  25,  1909. 

19.  Graefe  Arch.  f.  Ophth.  69,  p.  479. 

20.  Graefe  Arch,  f .  Ophth.  26,  p.  40. 

21.  Columbia.  Cont.  to  Phil,  and  Psych,  20,  No.  2. 

22.  Elec.  World,  1911,  58,  p.  1252;  Trans.  I.  E.  S.  1912,  p.  135. 

23.  Sci.  Amer.  Sup.  Feb.  2,  1913. 

24.  Jour,    de    Physiol.   et  de   Path.   Gen.  No.  4,  July,  1902; 

Comp.  Rend.  2,  1903,  p.  977,  p.  1046. 

25.  Phys.  Rev.  1914,  p.  1;  Elec.  World,  May  16,  1914. 

26.  Phil.  Mag.  1834,  5,  p.  327. 

27.  Physiol.. Optik.  II,  p.  483. 

28.  Pogg.  Ann.  d.  Phys.  1835,  35,  p.  457, 

29.  Pfliiger's  Archiv.  1878,  18,  p.  542. 

30.  Wied.  Ann.  1888,  34,  p.  465. 

31.  Phys.  Rev.  1895,  1,  p.  338. 


162  COLOR  AND  ITS  APPLICATIONS 

32.  Zeit.  Inst.  1896,  16,  p.  299. 

33.  Physiol.  der  Netzhaut,  p.  351. 

34.  Bui.  Bur.  Stds.  1905,  2,  p.  1. 

35.  Acad.  Sc.  Paris,  July  3, 1911;  Trans.  I.  E.  S.  1912,  7,  p.  625. 

36.  Comp.  Rend.  Soc.  Biol.  1885,  2,  p.  485. 

37.  Comp.  Rend.  Soc.  Biol.  1887,  2,  p.  5. 

38.  Proc.  Roy.  Soc.  1902,  79,  p.  313. 

39.  Jour,   of  Physiol.  21,  p.  126. 

40.  Phil.  Mag.  1912,  p.  352. 

41.  Meeting  Ry.  Signal  Assn.  1905. 

42.  Ann.  d .  Hydrographie,  1886. 

43.  Proc.  Phys.  Soc.  London,  1913,  24,  p.  379. 

44.  Elec.  World,  Dec.  6, 1913. 

45.  Elec.  World,  Jan.  15,  1912;    Trans.  I.  E.  S.  1914,  p.  472. 

46.  Proc.  Roy.  Soc.  London,  A,  214,  p.  1. 


CHAPTER  VII 

EFFECT  OF  ENVIRONMENT  ON  THE  APPEARANCE 
OF  COLORS 

41.  Colors  have  been  largely  treated  in  other 
chapters  as  if  they  were  invariable  in  appearance. 
However,  the  study  and  application  of  the  science  of 
color  is  rendered  very  complex  owing  to  the  fact 
that  the  appearance  of  a  color  is  so  modified  by  its 
environment.  The  intensity,  spectral  character,  and 
distribution  of  the  light  illuminating  it,  the  adaptation 
of  the  retina  for  light  and  color,  the  duration  of  the 
stimulus  and  the  character  of  the  stimulus  preceding 
the  one  under  consideration,  the  surroundings,  the 
size  and  position  of  the  retinal  image,  the  surface 
character  of  the  colored  medium,  all  affect  the  appear- 
ance of  a  given  color.  Thus  an  analysis  that  holds 
for  a  color  in  a  certain  environment  does  not  in 
general  hold  for  the  identical  colored  object  viewed 
under  other  conditions. 

The  size  of  a  colored  image  and  its  position  and 
duration  on  the  retina  affects  its  appearance,  owing 
to  the  variation  of  sensitivity  of  the  various  retinal 
zones.  MacDougal1  found  that  with  small  colored 
areas  (squares  from  1  to  16  sq.  cm.  in  area  viewed 
from  a  distance  of  one  meter)  the  larger  areas  ap- 
peared more  saturated  than  the  smaller.  He  found 
the  saturating  effect  of  increasing  the  area  greatest 
for  violet  and  decreasing  in  the  order,  blue,  green, 
yellow,  orange,  red.  He  even  concludes  that  a  color 
field  is  not  fully  saturated  until  it  extends  over  the 

163 


164  COLOR  AND   ITS  APPLICATIONS 

whole  field  of  vision.  This  can  hardly  be  true,  for 
an  observer  in  a  room  with  neutral  tint  surround- 
ings illuminated  with  pure  red  light  is  not  conscious 
of  a  saturated  red  color.  Similarly  if  a  white  paper 
on  a  black  velvet  ground  be  illuminated  by  a  moder- 
ately intense  red  light  it  will  appear  quite  unsaturated 
owing  to  the  lack  of  anything  with  which  to  contrast 
it  in  color.  The  loss  in  saturation  appears  to  progress 
with  time,  no  doubt  largely  due  to  'adaptation.' 
Whether  or  not  this  adaptation  is  psychological  or 
physiological  there  is  a  lack  of  agreement.  However, 
the  effect  of  area  is  of  importance,  although  there  is 
much  work  to  be  done  in  this  field  before  definite 
conclusions  can  be  drawn. 

Another  experiment  of  importance  in  viewing 
colors  which  is  connected  with  the  rate  of  growth  of 
color  sensations  and,  perhaps,  to  a  slight  degree, 
with  chromatic  aberration,  is  found  in  viewing  a  red 
piece  of  paper  on  a  blue-green  ground  held  at  an  arm's 
length  under  a  moderate  illumination.  If  the  paper 
be  moved  back  and  forth  without  relaxing  fixation  at 
a  point  in  the  plane  in  which  the  card  is  moved,  the 
red  patch  will  appear  to  shake  like  jelly  and  will 
appear  not  to  be  in  the  same  plane  as  the  blue-green 
paper.  Thus  there  are  numerous  visual  phenomena 
associated  with  the  appearance  of  colors. 

42.  Illumination.  —  It  has  already  been  shown 
that  the  maximum  spectral  sensibility  of  the  eye 
shifts  toward  the  shorter  wave-lengths  at  low  inten- 
sities (Purkinje  effect  #4,  Fig.  2).  Therefore  colors 
ordinarily  encountered  appear  to  shift  in  hue  under 
low  illumination.  For  example,  a  green  pigment 
appears  to  assume  a  more  bluish  hue  as  the  illumi- 
nation is  greatly  decreased.  On  referring  to  Fig.  2 
it  is  seen  that  the  relative  values  of  luminous  sensa- 


EFFECT   OF  ENVIRONMENT   ON   COLORS 


165 


tion  produced  by  equal  amounts  of  radiant  energy 
depend  upon  the  wave-length.  A  colored  pigment  has 
the  ability  to  reflect  certain  proportions  of  the  rays 
of  various  wave-lengths.  The  latter  is  a  purely  physi- 
cal operation  which  remains  invariable  regardless  of 
the  intensity  of  illumination.  However,  the  relative 
physiologic  effect  of  the  different  rays  change  so  that 
the  maximum  luminosity  is  produced  by  energy  of 
a  shorter  wave-length  at  low  intensities  than  at  high 
illumination.  By  multiplying  the  reflection  coefficients 


r\ 


0.50  Q54  058 

A,  WAVE  LENGTH 


0.4Z  0.46 

Fig.  79.  —  Effect  of  the  intensity  of  illumination  on  the  appearance  of  a  pigment. 


of  a  pigment  for  various  rays  by  the  luminosities  of 
the  corresponding  rays  at  high  and  low  intensities  an 
idea  of  the  shift  of  the  dominant  hue  is  obtained. 
This  was  done  for  a  green  pigment  by  using  the 
luminosity  curves  in  Fig.  2  for  high  (H)  and  low  (L) 
illumination.  The  results  plotted  with  equal  maxima 
are  shown  in  Fig.  79. 

Colors  appear  more  saturated  at  low  than  at  high 
intensities  of  illumination.  In  fact,  intense  illumina- 
tion causes  colors  to  appear  very  much  less  saturated. 
For  instance,  a  deep  red  object  illuminated  by  direct 
sunlight  is  painted  orange-red  by  the  artist.  The 
employment  of  this  illusion  is  successful  in  conveying 


166  COLOR  AND   ITS  APPLICATIONS 

to  the  observer  the  idea  of  intense  illumination.  Simi- 
larly colors  appear  more  saturated  when  exposed 
only  for  a  very  short  interval  of. time. 

Quality  or  spectral  character  of  light  affects  the 
appearance  of  colored  objects  very  much.  Except 
in  very  special  cases  a  red  fabric  for  example  appears 
red  because  it  has  the  ability  to  reflect  chiefly  the 
red  rays  (#  12,  Fig.  12).  Such  a  fabric  must  appear 
black  when  viewed  under  an  illuminant  which  con- 
tains no  red  rays.  This  is  the  case  under  the  light 
from  the  mercury  arc,  which  contains  practically  no 
visible  rays  longer  than  0.579^  (yellow).  It  is  a 
fundamental  principle  that,  excepting  in  special  cases, 
a  colored  fabric  cannot  appear  the  same  under  two 
different  illuminants.  Therefore  two  colors  that  ap- 
pear alike  under  one  illuminant  will  not  match  when 
viewed  under  another  illuminant,  unless  the  colors  in 
each  case  show  the  same  spectral  character  by  spec- 
trophotometric  analysis.  In  other  words,  because  the 
eye  is  not  capable  of  analyzing  a  color  spectrally,  it 
is  possible  to  produce  colors  which  appear  the  same 
but  whose  spectral  compositions  differ.  Such  a  match 
will  not  in  general  remain  a  match  under  another 
illuminant  differing  in  spectral  character.  In  Fig.  80 
the  effect  of  the  illuminant  upon  the  appearance  of 
a  colored  pigment  (purple)  is  shown  diagrammatically. 
The  relative  luminosities  (dotted  lines)  produced  by 
the  relative  amounts  of  energy  (full  lines)  of  cor- 
responding wave-lengths  are  shown  for  daylight  in 
the  illustration  on  the  left.  The  result  as  shown  by 
the  dotted  curve  is  to  give  to  the  pigment  the  appear- 
ance of  a  blue-purple.  However,  when  this  same 
fabric  is  illuminated  by  ordinary  artificial  light  of 
continuous  spectral  character,  the  excessive  amounts 
of  energy  of  the  longer  wave-lengths  and  the  defi- 


EFFECT   OF  ENVIRONMENT   ON   COLORS 


167 


ciency  in  short-wave  energy  as  compared  with  day- 
light alter  the  spectral  character  of  the  light  reflected 
(or  transmitted)  by  the  fabric  as  shown  in  the 
dotted  curve  on  the  right.  The  appearance  under  the 
artificial  light  is  red-purple.  It  is  difficult  to  distin- 
guish a  blue  fabric  as  blue  under  ordinary  artificial 
light  owing  to  the  scarcity  of  blue  rays  in  most  of 
the  artificial  illuminants.  Of  course  a  truly  mono- 
chromatic pigment  (if  such  existed)  would  not  be 


NOON  SUHLIGHT 


\ 


V     B      G     Y       OR 


BLUE -PURPLE 


ORDINARY  ARTIFICIAL  LIGHT 


V     B     G     Y     0      R 


RED-PURPLE 


Fig.  80.  —  Illustrating  why  a  purple  appears  differently  under  two  different 

illuminants. 


changed  in  hue  under  various  illuminants  but  would 
be  altered  in  brightness.  In  the  special  case  where 
no  energy  existed  in  the  illuminant  of  the  wave- 
length corresponding  to  that  <reflecte.d  by  the  mono- 
chromatic pigment,  the  latter  would  appear  black. 
However,  no  monochromatic  colors  are  found  in 
practise,  but  if  pigments  that  were  practically  mono- 
chromatic existed  very  generally,  a  greater  intensity 
of  illumination  would  very  often  be  required  than 
at  present,  because  such  colors  would  reflect  very 
little  light.  Pigments  ordinarily  encountered  re- 
flect considerable  light,  owing  to  the  fact  that  they 


16* 


COLOR  AND   ITS  APPLICATIONS 


reflect    energy    throughout    an    appreciable    range    of 
wave-lengths. 

The  spectral  character  of  an  illuminant  not  only 
influences  the  hue  but  also  affects  the  brightness  or 
*  value'  of  a  pigment.  Practically  the  whole  series 
of  Zimmerman  papers  were  measured  for  their  rela- 
tive brightness  with  a  reflectometer  (Fig.  60)  under 
illuminations  respectively  from  an  overcast  sky  and 
from  a  vacuum  tungsten  lamp  operating  at  7.9  lumens 
per  watt.  The  data  are  given  in  Table  XV,  the  cata- 

TABLE   XV 
Effect  of  Spectral  Character  of  Light  on  the  Brightness  of  Colored  Papers 


Reflection 

Coefficient 

Tungsten 

.  R  (Tungsten) 

Paper 

Color  of  paper 

Overcast  sky. 

1.25 
w.  p.  m.  h.  c. 

tl0R  (Skylight) 

a 

Red-purple  

0.16 

0.23 

1.44 

b 
c 

Deep  red  
Red          .               

.14 
.21 

.22 
.31 

1.57 
.48 

d 

Red 

.19 

.24 

.22 

e 
f 

Orange  
Orange-yellow  
Yellow 

.38 
.60 
.60 

.48 
.66 
.65 

.26 
.10 
.08 

h 
k 
i 

Greenish  yellow  
Yellow-green  
Dull  green  
Saturated  green  

.67 
.46 
.49 
.32 

.70 
.42 
.45 
.24 

.04 
0.91 
0.92 
0.75 

Q 

Blue                  

.23 

.17 

0.74 

Q 

Deep  blue 

.13 

.09 

A  0.69 

Blue-purple.  .  .r  

r       .14. 

.12 

9 
0.86 

m 

Gray-blue  

.30 

.25 

0.83 

logue  designations  of  the  papers  being  found  in  the 
first  column.  As  would  be  suspected,  the  papers 
which  have  the  ability  to  reflect  the  rays  of  longer 
wave-length  predominantly  appear  relatively  brighter 
under  the  artificial  light.  In  other  words  their  reflec- 
tion coefficients  are  greater  for  the  tungsten  light 


EFFECT   OF  ENVIRONMENT   ON   COLORS  169 

than  for  daylight.  Those  colors  having  the  ability 
to  reflect  the  rays  of  shorter  wave-length  predomi- 
nantly, have  relatively  greater  reflection  coefficients 
for  daylight;  that  is,  they  appear  relatively  brighter 
under  daylight  illumination.  (See  Figs.  113  and  117.) 
Thus  it  is  seen  that  the  spectral  character  of  the 
illuminant  has  a  great  influence  on  the  appearance 
of  a  colored  object,  inasmuch  as  it  influences  both 
the  hue  and  brightness  (value)  very  much. 

Owing  to  the  surface  character  of  colored  media 
the  distribution  of  light  is  of  some  importance  in  the 
consideration  of  the  appearance  of  colors.  Few  pig- 
ments are  applied  in  such  a  manner  as  to  be  per- 
fectly diffusing,  therefore  some  light  is  specularly 
reflected  without  having  penetrated  the  pigment. 
This  light  is  unchanged  by  selective  absorption  and 
dilutes  the  light  that  is  colored  by  penetrating  the 
pigment  and  being  selectively  reflected.  That  is, 
when  the  light  is  distributed  in  such  a  manner  that 
an  appreciable  amount  is  specularly  reflected  into 
the  eye  of  the  observer  the  color  appears  less  satu- 
rated. In  the  extreme  case  of  high  specular  reflection 
the  pigment  appears  the  same  as  a  gray.  A  striking 
illustration  of  the  effect  of  distribution  of  light  is 
found  in  the  case  of  the  so-called  changeable  silks. 
Such  fabrics  have  a  nap,  and  when  the  fibers  end  in 
the  direction  toward  the  light  the  latter  penetrates 
the  fabric  and  is  deeply  colored  by  multiple  selective 
reflections.  The  light  that  comes  from  other  direc- 
tions is  more  or  less  specularly  reflected,  thus  under- 
going less  change  by  selective  absorption,  with  the 
result  that  various  portions  of  the  surface  appear 
differently.  Adding  to  the  foregoing  another  property 
of  aniline  dyes  and  the  colors  of  changeable  silks  are 
accounted  for.  For  instance,  a  dye  which  in  solu- 


170  COLOR  AND   ITS  APPLICATIONS 

tion  appears  pink  or  purple  in  color  is  often  found  to 
reflect  green  light  predominantly  in  the  solid  state. 
Thus  the  specularly  reflected  light  in  the  case  of  the 
changeable  silk  is  sometimes  roughly  complementary 
to  the  light  that  penetrates  the  fabric  and  is  returned 
after  multiple  reflections  which  in  effect  correspond 
to  traversing  a  certain  depth  of  an  aqueous  solution 
of  the  dye.  A  color  will  often  appear  different  by 
reflection  than  when  examined  *  over-hand'  by  look- 
ing through  the  fibers  by  glancing  along  the  surface 
at  a  grazing  angle  (#75). 

\l  43.  After-images.  —  It  has  been  seen  that  the 
retinal  excitation  requires  appreciable  time  to  decay 
after  the  stimulus  has  been  removed.  If  the  filament 
of  an  incandescent  lamp  be  viewed  for  an  instant  and 
the  eye  be  then  closed,  an  image  brighter  than  the 
surroundings  will  persist  for  some  time.  This  has 
been  called  a  positive  after-image.  Soon,  depending 
upon  the  intensity  of  the  stimulus,  the  image  will 
reach  a  stage  of  decay  when  it  appears  darker  than 
the  surroundings.  If  the  closed  eyelid  be  illumi- 
nated the  visual  field  will  appear  brighter  than  in  the 
case  where  the  eyelid  is  shielded  from  the  light  by 
the  hand  placed  gently  against  it.  In  the  former 
case  the  after-image  will  remain  'positive'  a  shorter 
time  than  in  the  case  of  the  darker  surroundings. 
The  same  will  be  found  when  viewing  the  after- 
image against  various  white  or  gray  backgrounds 
with  the  eyelid  open.  The  effect  of  the  brightness 
of  and  exposure  to  the  stimulus  on  the  duration  of 
the  after-image  is  shown  in  Fig.  81.  In  this  experi- 
ment the  actual  duration  was  somewhat  longer  than 
indicated,  the  criterion  being  the  time  after  exposure 
that  was  required  for  the  after-image  to  decay  to  a 
certain  definite,  though  faint,  appearance-  The  after- 


EFFECT  OF  ENVIRONMENT   ON   COLORS 


171 


images  were  found  to  go  through  a  certain  cycle  of 
brightness  and  hue  changes  which  were  not  recorded. 
The  stimulus  was  a  bare  tungsten  filament  varied  in 
brightness  by  a  variable  sectored  disk  which  was 
rotated  at  a  high  speed.  The  brightness  is  given  in 
terms  of  candles  per  square  inch.  The  after-images 
were  observed  against  a  faint  background  produced 
by  the  illumination  of  the  closed  eyelid  by  a  small 
amount  of  stray  light  in  the  room.  The  changes  in 


EXPOSURE  (SECONDS) 
Fig.  81.  —  Effect  of  brightness  on  the  duration  of  the  after-image. 

hue  were  not  recorded.  Positive  after-images,  ob- 
tained by  fixating,  for  a  few  seconds,  a  white  paper 
illuminated  by  sunlight,  can  be  seen  for  a  brief 
period,  but  they  rapidly  decay  to  a  brightness  lower 
than  that  of  ordinary  surroundings.  In  their  decay 
they  pass  through  a  series  of  hues,  namely  blue- 
green,  indigo,  violet-pink,  dark  orange,  etc.,  which 
are  more  or  less  definite.  Helmholtz  explains  the 
colored  after-images  obtained  in  the  above  manner 
by  assuming  different  rates  of  decay  of  the  three 
hypothetical  color  sensations  which  are  the  basis  of 


172  COLOR  AND   ITS   APPLICATIONS 

the  Young-Helmholtz  theory  of  color  vision  (#47). 
The  ^negative  after-image  is  explained  by  Helmholtz 
as  being  caused  by  retinal  fatigue,  due  to  the  origi- 
nal bright  image  of  the  white  object.  On  stimu- 
lating the  whole  retina  with  white  light  the  portion 
previously  fatigued  does  not  respond  in  the  same 
degree  as  the  unfatigued  portions.  It  is  difficult, 
however,  to  reconcile  all  the  facts  gleaned  from 
studies  of  after-images  with  this  fatigue  hypothesis. 
Hering  explains  these  phenomena  by  assuming  that 
the  retina  is  not  fatigued,  but  that  a  metabolic  change 
is  aroused  which  is  opposite  in  character  to  that  pro- 
duced by  the  original  excitation  (#  49).  After-images 
are  also  produced  by  fixating  colored  objects.  For 
instance,  if  the  object  shown  in  Fig.  85  be  fixated 
for  a  few  seconds  and  the  eye  be  turned  toward  a 
white  surface,  a  pink  after-image  will  be  seen  where 
the  green  had  previously  stimulated  the  retina.  After- 
images viewed  in  this  manner  will  usually  appear 
approximately  complementary  in  hue  to  the  original 
stimulus.  A  striking  illustration  of  approximately 
complementary  after-images  can  be  performed  with 
the  apparatus  shown  in  Fig.  35.  Here  we  have  a 
large  variety  of  colors,  the  corners  of  the  triangle 
appearing  red,  green,  and  blue.  After  fixating  the 
color  triangle  for  a  few  seconds,  if  the  lights  be  turned 
off  and  white  light  be  permitted  to  illuminate  the 
white  opal-glass  surface,  approximately  complemen- 
tary after-images  are  seen.  The  experiment  is  strik- 
ing, owing  to  the  many  colors  present.  In  order  to 
produce  the  complementary  after-images  in  a  striking 
manner,  it  is  necessary  to  stimulate  the  retina  with 
white  light.  If  the  experiment  be  performed  in  a 
dark  room  after  the  lights  ill  the  colored  triangle  are 
extinguished,  the  ordinary  after-images  will  be  per- 


EFFECT  OF  ENVIRONMENT  ON   COLORS  173 

ceived,  depending  upon  the  color  of  the  stimulus, 
its  intensity,  and  other  factors.  After-images  play 
quite  an  important  part  in  vision,  especially  in  viewing 
paintings  and  many  other  colored  objects.  For  in- 
stance, if  a  blue  skyline  be  viewed  in  juxtaposition 
with  a  green  landscape,  the  natural  shifting  of  the 
eye,  even  when  attempting  moderately  to  gaze  steadily 
at  the  picture,  will  cause  a  shifting  of  the  image  of 
this  dividing  line  upon  the  retina,  with  the  result  that 
the  pinkish  after-image  due  to  the  green  stimulus 
(and  likewise  that  due  to  the  blue  stimulus)  will,  in 
shifting  above  and  below  the  horizon  line,  produce 
a  vivid  effect.  Such  phenomena  often  greatly  add  to 
the  'life'  of  a  painting.  After  steadily  fixating  a 
colored  object  for  some  time  the  color  appears  to 
become  less  saturated,  and  often  there  is  an  apparent 
change  in  hue.  If  a  small  piece  of  black  paper  on 
a  larger  background  of  red  be  fixated  for  a  few 
seconds  and  the  black  paper  be  suddenly  removed 
without  disturbing  the  fixation,  in  its  place  will  be 
seen  a  red  spot  more  luminous  than  the  surroundings 
and  of  a  more  saturated  reddish  appearance.  These 
experiments  can  be  successfully  performed  with  the 
Zimmerman  colored  papers. 

Successive  contrast  furthei;  complicates  the  appear- 
ance  of  colors.  After  stimulating  the  retina  with 
red  light,  if  the  eye  be  suddenly  fixated  upon  a 
green  color  the  latter  will  appear  more  intense  or 
saturated  in  color  for  a  moment  than  if  the  eye  had 
not  been  previously  stimulated  by  the  red  light.  On 
alternating  these  colors  by  means  of  a  rotating  disk 
at  a  low  speed  very  brilliant  effects  are  seen.  Such 
successive  contrasts  are  of  importance  in  the  study 
and  application  of  color  science.  For  instance,  in 
permitting  the  eye  to  rove  over  a  painting  or  bril- 


174 


COLOR  AND  ITS  APPLICATIONS 


v 


liantly  colored  rug  the  appearance  of  the  various 
individual  colors  are  influenced  by  the  previous 
retinal  stimulation.  The  phenomenon  of  after-images 
has  been  only  briefly  touched  upon  here.  The  many 
details  connected  with  them  serve  to  show  how  intri- 
cate is  the  reaction  of  the  retina  to  light. 

44.  Simultaneous  Contrast.  —  A  detailed  exami- 
nation of  the  mutual  effect  of  two  visual  excitations 
is  difficult,  although  the  fundamental  principles  are 


Fig.  82.  —  Showing  the  effect  of  simultaneous  contrast.    The  V's  are  of  equal 

brightness. 

not  difficult  to  demonstrate.  On  viewing  a  gray 
pattern  on  a  black  background  it  appears  brighter  than 
when  viewed  upon  a  light  background.  The  illus- 
tration shown  in  Fig.  82  was  originally  made  by  cut- 
ting the  two  figures  in  the  form  of  a  V  from  the  same 
gray  paper.  On  placing  them  as  shown  —  one  on  a 
black  and  one  on  a  white  ground  —  the  one  on  the 
white  ground  appears  darker  than  the  other.  The 
effect  is  so  persistent  that  a  much  darker  gray  can 
be  placed  on  the  black  background  and  yet  it  will 
appear  brighter  than  the  one  on  the  white  ground. 


EFFECT  OF  ENVIRONMENT  ON  COLORS          175 

In  fact  it  is  practically  impossible  to  make  both  appear 
alike  by  decreasing  the  brightness  of  the  gray-  V  on 
the  dark  ground.  If  several  gray  papers  of  different 
shades  be  placed  as  shown  in  Fig.  83,  the  edge  of  a 
lighter  gray  strip  that  is  adjacent  to  a  darker  one 
will  appear  brighter  than  the  other  edge  of  the  lighter 
gray  strip.  Such  a  specimen  can  be  obtained  by 
juxtaposing  gray  papers  of  different  shades  or  by 
exposing  a  photographic  plate  in  a  very  weak  light 
by  pulling  out  the  slide  of  the  plate  holder  a  half- 
inch  at  a  time  at  regular  intervals.  A  print  from  a 


Fig.  83.  —  Showing  induction.    Each  band,  though  uniform  in  brightness,  appears 
brighter  at  the  right-hand  edge. 

successful  negative  will  afford  an  excellent  specimen 
for  showing  this  phenomenon  of  induction.  On  rotat- 
ing such  a  disk  shown  in  6,  Fig.  30,  this  phenomenon 
of  induction  can  be  demonstrated  before  a  large 
audience.  A  striking  demonstration  of  brightness 
contrast  can  be  performed  by  viewing  a  gray  paper 
through  a  hole  in  a  white  unilluminated  screen.  It 
appears  very  bright  in  contrast  to  the  dark  surround- 
ings. However,  on  illuminating  the  white  surround- 
ings it  is  possible  to  make  the  former  bright  spot 
appear  very  dark  by  contrast.  The  demonstrations 
of  color  contrast  are  very  numerous.  M.  E.  Chev- 
reul,2  who  directed  the  dyeing  laboratory  of  the 
famous  Gobelins  many  years  ago,  carried  out  very 
extensive  researches  on  the  effect  of  simultaneous 
contrast  of  colors  as  used  in  the  textile  industry.  The 


176 


COLOR  AND   ITS  APPLICATIONS 


record  of  his  experiments  is  monumental.  Many 
have  investigated  the  problem  with  the  view  of  throw- 
ing light  on  color-vision  theories,  but  many  of  the 
details  garnered  by  the  vast  number  of  investigators 
remain  unsatisfactorily  explained. 

The  intensity  of  the 
contrast  effect  diminishes 
rapidly  on  passing  away 
from  the  point  of  maximum 
contrast.  In  Fig.  84  when 
two  colors  such  as  red  and 
green  are  juxtaposed  they 
appear  accentuated  in  satu- 
ration and  deeper  in  hue. 
In  the  case  of  these  two 


Green 

n 


Fig.   84.  —  An    arrangement    for     Colors    they    appear    to    move 

cttTs?  etct  $"££*£  £    further  a?**  *  hue'    Wh«* 
two  colored  objects.  the  two  colors  are  separated 

as  shown  above,  the  con- 
trast effect  practically  disappears.  If  a  disk  of  green 
be  placed  on  a  larger  disk  of  red  the  contrast  is  very 
effective  but  if  the  smaller  disk 
is  outlined  by  a  black  circle  the 
effect  is  reduced.  If  a  gray  figure, 
as  in  Fig.  85,  be  placed  upon 
a  green  background,  the  gray 
figure  will  appear  of  a  pink 
hue.  The  contrast  hue  induced 
in  this  manner  is  approxi- 
mately, though  in  general  not 
exactly,  complementary  to  the 
exciting  color.  If  a  sheet  of 
thin  white  tissue  paper  be  placed 
over  the  arrangement  shown  in  Fig.  85,  the  hue  induced 
in  the  gray  paper  will  be  considerably  strengthened. 


G  roy 


Fig.  85.  —  An  arrangement 
for  showing  the  effect  of 
simultaneous  contrast  and 
after-images. 


EFFECT  OF  ENVIRONMENT   ON   COLORS  177 

Colored  shadows  were  noticed  by  such  great 
colorists  as  Leonardo  da  Vinci.  These  are  illus- 
trated by  casting  the  shadow  of  a  pencil  on  white 
paper  by  light  entering  a  window;  only  black  and 
white  contrast  is  seen.  However,  if  from  another 
direction  light  from  an  incandescent  lamp  be  per- 
mitted to  fall  on  the  paper,  another  shadow  is  pro- 
duced. If  the  two  shadows  are  of  approximately 
the  same  brightness,  the  contrast  colors  of  the  shad- 
ows are  very  striking.  The  white  ground  outside 
the  shadows  is  receiving  the  mixed  light  from  the 
two  illuminants,  while  the  shadow  cast  by  daylight 
receives  light  from  only  the  incandescent  lamp  and 
appears  yellow.  The  other  shadow  receiving  only 
daylight  appears  blue.  Shadows  in  a  landscape 
appear  blue  because  they  receive  light  from  the  sky, 
and  they  often  appear  more  vivid  owing  to  contrast. 
Hering  devised  a  most  striking  demonstration  of 
binocular  contrast.  Red  and  blue  glasses  were  placed 
in  front  of  the  two  eyes  respectively.  The  glasses 
sloped  away  from  the  eyes  from  the  nasal  to  the 
temporal  side.  This  permitted  a  control  of  satura- 
tion by  introducing  a  white  image  from  the  sides  by 
reflection.  A  black  stripe  on  a  white  ground  is 
doubled  by  increasing  or  decreasing  the  ocular  diver- 
gence. The  observed  ground  appears  spotted,  alter- 
nately blue  and  red,  and  sometimes  a  purplish  white, 
which  is  due  to  'retinal  rivalry.'  The  stripe  seen 
through  the  red  glass  appears  green  and  through  the 
blue  glass  appears  yellow. 

Briicke,  Helmholtz,  and  others  contend  that  the 
contrast  effects  are  not  of  a  physiological  nature,  but 
rather  'errors  of  judgment';  that  is,  through  the  in- 
fluence of  an  adjacent  color  our  'standard  white' 
undergoes  a  change  which  alters  our  judgment.  In 


178  COLOR  AND   ITS  APPLICATIONS 

other  words  they  claim  the  effect  is  of  a  psychological 
nature.  These  arguments  have  been  repeatedly 
attacked  and  not  without  a  considerable  degree  of 
success  by  Hering  and  others.  For  example,  Mayer  3 
devised  methods  for  showing  contrast  color  phe- 
nomena on  surfaces  large  enough  so  that  the  colors 
could  be  matched  by  means  of  rotating  color  disks 
and  thus  he  obtained  quantitative  measurements. 
He  found  that  the  subjective  contrast  colors  were 
perceptible  when  viewed  through  a  small  opening 
for  exposures  as  short  as  0.001  second.  They  were 
also  perceptible  with  instantaneous  illumination  from 
an  electric  spark,  the  duration  of  illumination  in 
this  case  being  of  the  order  of  one  ten-millionth  of  a 
second.  He  concluded  from  this  experiment  that 
fluctuation  of  judgment  was  an  untenable  hypothesis 
for  explaining  subjective  color  contrast  owing  to  the 
extremely  short  period  of  time  of  exposure. 

On  the  other  hand,  Edridge-Green 4  contends  'that 
all  our  estimations  of  color  are  only  relative  and  formed 
in  association  with  memory  and  the  definite  objec- 
tive light  which  falls  upon  the  eye.  In  many  of  the 
most  striking  contrast  experiments  the  color  which 
causes  the  false  interpretation  is  not  perceived  at 
all;  for  instance,  if  a  sheet  of  pale  green  paper  be 
taken  for  white,  a  piece  of  gray  paper  upon  it  appears 
rose  colored,  but  appears  colorless  when  it  is  recog- 
nized that  the  paper  is  pale  green  and  not  white.' 

Thus  the  controversy  continues.  Many  contra- 
dictory experimental  data  and  opinions  could  be  cited. 
Contrast  may  be  due  to  unconscious  eye-movements, 
to  incipient  retinal  fatigue,  to  fluctuation  or  error  of 
judgment,  or  to  some  other  cause.  Nevertheless 
there  is  no  agreement  as  to  the  true  explanation  at 
the  present  time. 


EFFECT  OF  ENVIRONMENT  ON  COLORS  179 

In  Plate  III  are  provided  a  number  of  arrange- 
ments which  show  the  effects  of  simultaneous  con- 
trast —  brightness  and  hue  contrasts  —  and  various 
mixtures  of  these.  Some  of  these  illustrate  restful 
and  'lively'  combinations  of  color.  There  will  be 
found  much  of  interest  in  this  illustration  upon  care- 
fully observing  the  various  combinations  alone  and 
in  comparison  with  adjacent  ones.  The  four  smaller 
squares  in  each  row  are  identical  in  hue  and  bright- 
ness, which  can  be  readily  proved  by  the  use  of  a 


Fig.  86. — Illustrating  irradiation. 

mask.     If  this  plate  be  covered  with  a  white  tissue 
paper  some  of  the  color  contrasts  are  very  striking. 

45.  Irradiation.  —  This  name  is  applied  to  the 
phenomenon  of  apparent  increase  in  size  of  objects 
as  they  are  increased  in  brightness.  For  instance, 
the  crescent  of  the  new  moon  appears  larger  than 
the  remainder  of  the  disk.  A  filament  of  an  incandes- 
cent lamp  appears  to  increase  in  diameter  as  its 
temperature  is  raised  from  a  dull  red  to  its  normal 
operating  temperature.  This  effect  has  been  attrib- 
uted by  some  to  a  spreading  of  the  retinal  image  on 
account  of  a  stimulation  of  nerves  outside  its  actual 
geometric  boundary.  Others  attribute  the  effect  to 


180  COLOR  AND   ITS  APPLICATIONS 

the  aberrations  in  the  optical  system  of  the  eye.  In 
Fig.  86  the  inner  white  square  appears  larger  than 
the  inner  black  square  under  high  illumination,  yet 
both  are  identical  in  size.  The  phenomena  of  simul- 
taneous brightness  contrast  is  also  evident,  the  white 
square  amid  black  surroundings  appearing  brighter 
than  the  larger  white  square.  Such  effects  are  also 
perceptible  with  colored  objects,  as  will  be  seen  in 
Plate  III. 

REFERENCES 

1.  Amer.  Jour,  of  Psych.  13,  p.  481. 

2.  The  Principles  of  Harmony  and  Contrast  of  Colours,  1860. 

3.  Amer.  Jour,  of  Sci.,  July,  1893. 

4.  Proc.  Roy.  Soc.  B,  1913,  86,  p.  110. 

OTHER    REFERENCES 

Tschermak,  Ueber  das  Verhaltnis  von  Gegenfarbe,  Kompen- 
sationsfarbe  und  Kontrastfarbe,  Phlug.  Arch.  1907,  117,  p.  204. 

F.  Klein,  Nachbilder,  Uebersicht  und  Nomenklatur,  Englemann's 
Arch.  f.  Physiol.  1908,  Sup.  Bd.  p.  219. 

G.  J.  Burch,  Proc.  Roy.  Soc.  1900,  66,  p.  204. 

An  excellent  bibliography  of  the  work  on  simultaneous  contrast 
is  given  by  A.  Tschermak,  Ueber  Kontrast  und  Irradiation,  Ergeb- 
nisse  d.  Physiol.  1903,  p.  726. 

General  references  are  Helmholtz,  Handbuch  d.  Physiol.  Optik 
and  Nagel's  Handbuch. 


CHAPTER   VIII 
THEORIES   OF  COLOR  VISION 

46.  Recorded   writings,   centuries   before   the   be- 
ginning of  the  Christian  era,  contain  speculations  on 
the  visual  process.      Alcmaeon,  Empedocles,  Aristotle, 
Derhocritus,    Anaxagoras,    Plato,    and    Diogenes    are 
among  the   early  writers  and  philosophers  who  pre- 
sented views  on  the  nature  of  light  and  colors  and 
on   the    process   of  vision.      However,   their   specula- 
tions —  which   can    hardly  be    considered    otherwise, 
owing    to    lack    of    experimental    data  —  are    of    little 
value  since  the  modern  development  of  the  sciences. 
Color  vision   is  largely  physiological  and  psychologi- 
cal.     The    process    of    vision    involves    the    physical 
cause,  the  physiological  retinal  process,  and  the  psy- 
chological  elements  in  the   experience  of  sensations. 
As  the  knowledge  of  the  three  sciences  involved  in 
the    process    of    color   vision    developed,    theories    of 
color  vision  became  more  intricate.     In  fact  the  vari- 
ous theories  which  are  given  credence  at  the  present 
time  are  found  on  strict  analysis  to  include  in  vary- 
ing  degrees   the   physiologic  process   of   vision,   color 
vision,    and   the   nature    of   perception.     A   theory   of 
color   vision   must   include    all   the  foregoing  factors, 
yet  the  dominating  influence  of  one  of  these  is  usually 
perceptible  in  a  given  theory.     In  this  chapter  it  is 
proposed  to  set  forth  briefly  the  latest  theories  which 
pertain  to  the  subject  of  color  vision. 

47.  Young-Helmholtz    Theory. — Thomas    Young 
is    credited    with   the    conception    of   the    three-color 

181 


182  COLOR  AND   ITS  APPLICATIONS 

theory,  but  it  seriously  lacked  experimental  founda- 
tion until  after  the  epoch-making  work  of  Helm- 
holtz,1  and  since  that  time  it  has  become  known  as 
the  Young-Helmholtz  theory.  The  hypothesis  is  that 
color  sensations  depend  upon  the  action  of  three 
independent  physiological  processes  involving  three 
substances  or  sets  of  nerves.  This  theory  approaches 
the  matter  chiefly  from  the  side  of  physics;  that 
is,  the  facts  of  color-mixture  are  used  in  building  up 
the  theory.  There  is  no  anatomical  evidence  that  the 
three  substances  or  sets  of  nerves  are  present.  The 
primary  sensation  curves  shown  in  Fig.  53  were 
determined  by  Koenig  by  being  built  up  from  experi- 
mental data;  these  have  been  proposed  as  repre- 
senting the  three  independent  processes.  They  are 
plotted  so  as  to  enclose  equal  areas  on  the  assumption 
that  the  sensation  of  white  results  from  the  stimula- 
tion of  equal  amounts  of  the  three  primary  sensations. 
It  is  noted  that  spectral  hues  involve  more  than  one 
of  the  primary  sensations. 

This  theory  explains  the  main  facts  of  color  vision, 
although  many  details  uncovered  by  experimenters 
have  not  yet  been  reconciled  with  it  to  the  entire 
satisfaction  of  many  scientists.  After-images  are 
explained  by  assuming  fatigue  of  one  or  more  of  the 
processes  in  varying  degrees.  For  instance  after 
fatiguing  the  eye  to  green  light  a  white  surface 
appears  an  unsaturated  purple  —  pink.  Many  of 
the  observed  facts  in  the  study  of  after-images  are 
only  approximately  conciliable  with  this  theory.  The 
problem  of  simultaneous  contrast  offered  no  diffi- 
culties to  Helmholtz,  because  he  assumed  that  the 
phenomenon  is  the  result  of  'false  judgment.'  While 
it  may  be  purely  psychological,  it  appears  probable 
to  some  that  it  is  actually  physiological  in  nature, 


THEORIES   OF  COLOR  VISION  183 

one  part  of  the  retina  being  influenced  by  stimulation 
of  another  region.  Color-blindness  is  explained  by 
assuming  that  one  or  more  of  the  three  processes  are 
absent,  the  remaining  process  (or  processes),  if 
necessary,  being  assumed  to  be  'redistributed'  to 
some  extent.  This  theory  has  some  advantages  in 
explaining  the  cases  of  red  and  of  green  blindness 
by  assuming  the  absence  of  the  corresponding  process 
and  if  necessary  a  slight  modification  of  the  other 
two.  It  fails  to  explain  total  color-blindness,  however. 
When  it  is  attempted  to  reconcile  this  theory  with 
all  the  observed  facts,  one  finds  a  highly  complex 
state  of  affairs.  Such  a  discussion  is  outside  the 
scope  of  this  chapter,  therefore  only  the  main  theories 
and  facts  will  be  presented.  Extended  discussions 
will  be  found  in  the  treatises  referred  to  at  the  end 
of  this  chapter.  The  Young-Helmholtz  theory  satis- 
factorily explains  the  observed  facts  of  color-mixture, 
but  the  chief  objection  to  the  hypothesis  as  it  exists 
at  the  present  time,  is  that  it  fails  to  explain  many 
other  facts,  such  as  those  of  contrast. 

48.  'Duplicity'  Theory.— This  theory,  which  at- 
tempts to  differentiate  colorless  and  color  vision,  is 
chiefly  associated  with  the  name  of  Von  Kries.  It 
is  based  upon  anatomical  evidence  of  the  existence 
of  'rods'  and  'cones'  in  the  retina.  The  former  are 
assumed  to  be  responsible  for  achromatic  sensations 
and  the  latter  for  both  achromatic  and  chromatic  sen- 
sations. The  rod  action  is  supposed  to  be  largely 
responsible  for  ,  light  sensation  at  twilight  illumina- 
tion and  is  in  general  more  responsive  to  rays  of 
shorter  wave-length.  The  cones,  however,  are  sup- 
posed only  to  act  under  stimuli  of  brightnesses  repre- 
sented by  the  range  above  twilight  illumination  and 
not  to  be  greatly  increased  in  sensitiveness  by  dark 


184  COLOR  AND   ITS  APPLICATIONS 

adaptation.  Examination  of  the  retina  shows  that 
the  cones  alone  exist  in  the  very  center  of  the  retina, 
the  fovea  centralis,  and  rods  appear  just  outside  of 
this  and  predominate  in  the  outer  zones.  The  chief 
observed  facts  that  this  theory  explains  fairly  satis- 
factorily (perhaps  because  it  was  chiefly  built  up 
from  these  facts)  are  (1)  colorless  vision  over  the 
whole  retina  in  dim  light,  for  instance  in  moonlight, 

(2)  the  decreased  sensitivity  of  the  fovea  in  twilight, 

(3)  the  shift  in  the  maximum  of  the  luminosity  curve 
of  the  eye  (Purkinje   effect)  at  low  illumination,  (4) 
the  absence  of  such  a  shift  for  foveal  vision,  (5)  no 
achromatic  threshold  is  found  for  any  light  for  foveal 
vision,  (6)  no  achromatic  threshold  for  red  light  for 
any  region  of  the  retina,  and  (7)  colorless  vision  over 
the  whole  retina  in  the  case  of  the  totally  color  blind. 
Some  of  the   experiments  with  color-blind  eyes  fur- 
ther support  the  theory.     For  instance  the  luminosity 
curve  for  a  totally  color-blind  eye  at  ordinary  illumi- 
nations is  similar  to  that  for  a  normal  eye  for   twi- 
light vision.     There  are  also  evidences  of  diminished 
foveal    sensibility,    abnormally    good    vision    in    twi- 
light,   and   decreased   ability   to   fixate    small   objects 
with   color-blind   eyes.     Further   support  is  found   in 
the  presence  of  rods  almost  exclusively  in  the  retinae 
of  such  nocturnal  animals  as  the  owl  and  bat.     The 
supporting    evidence    in    general    is    represented    by 
more    dependable    and   convincing   data   than   in   the 
case   of   any   theory   of   color- vision.      The    'duplicity 
theory*  does  not  attempt  to  explain  color-vision,  but 
is  of  interest  here  because  of  the  attempt  to  separate 
vision  into  chromatic  and  achromatic  processes. 

49.  The  Hering  Theory.  -  -  The  principal  foun- 
dation of  this  theory  2  consists  of  facts  such  as  those 
of  contrast,  and  the  apparent  simplicity  of  black, 


THEORIES  OF  COLOR  VISION  185 

white,  and  yellow  as  well  as  red,  green,  and  blue. 
Hering  assumes  there  are  six  fundamental  sensa- 
tions coupled  in  pairs,  namely,  white  and  black,  red 
and  green,  yellow  and  blue.  In  order  to  account  for 
these  six  fundamental  sensations  he  assumes  the 
presence  somewhere  in  the  retinocerebral  apparatus 
of  three  distinct  substances.  Each  substance  is 
capable  of  building  up  (anabolism)  or  of  breaking 
down  (katabolism)  under  the  influence  of  radiant 
energy  or  its  effects.  The  building  up  of  the  black- 
white  substance  causes  a  sensation  of  blackness,  and 
the  breaking-down  of  this  substance,  a  sensation  of 
whiteness.  Likewise  anabolism  of  the  red-green 
substance  is  connected  with  the  sensation  of  green 
and  katabolism  with  red  sensation.  Similarly,  the 
building  up  of  the  third  substance  produces  blue, 
and  the  breaking  down  is  associated  with  yellow 
sensation.  For  example,  red  rays  cause  a  breaking 
down  of  the  red-green  substance,  with  the  result  that 
red  sensation  is  experienced.  It  is  claimed  by  many 
that  this  theory  has  an  advantage  over  the  Young- 
Helmholtz  theory,  because  it  deals  more  directly  with 
the  sensations  of  color.  The  theory  has  many  en- 
thusiastic supporters  and  is  fully  as  favored  in  this 
respect  as  its  most  formidable  rival.  A  favorite 
argument  in  support  of  it  is  the  observed  fact  that 
yellow  appears  to  be  a  primary  color  because  there 
is  no  simultaneous  suggestion  of  both  red  and  green 
in  a  yellow  made  by  mixing  these  two  colors  (Fig. 
17).  Many  observed  facts  concerning  after-images 
agree  with  the  theory.  For  example,  if  the  eye  be 
stimulated  by  blue  rays,  anabolism  will  take  place  in 
the  yellow-blue  substance  and  an  accumulation  of 
the  substance  results.  If  now  yellow  rays  are  per- 
mitted to  stimulate  the  same  area  of  retina,  the  break- 


186  COLOR  AND  ITS  APPLICATIONS 

ing  down  of  the  yellow-blue  substance  proceeds  at  a 
greater  rate  and  the  sensation  is  greatly  augmented. 
Conversely  yellow  decreases  the  amount  of  substance 
and  increases  the  rate  of  anabolism  under  the  sub- 
sequent stimulation  of  blue  rays.  Positive  after- 
images are  explained  by  assuming  a  continuation 
of  the  anabolic  (or  katabolic)  change  for  a  brief  period 
owing  to  chemical  inertia.  All  the  general  phenomena 
of  after-images  are  explained  satisfactorily,  but  as  in 
the  case  of  the  Young-Helmholtz  theory,  details  are 
troublesome.  Some  of  the  data  on  color-blindness 
readily  support  the  theory,  but  the  latter  must  be 
modified  in  order  to  explain  other  data.  Donders  3 
and  others  conclude  that  the  Young-Helmholtz  and 
Hering  theories,  having  been  formulated  from  dif- 
ferent points  of  view,  have  arrived  at  different  con- 
clusions and  that  both  are  in  part  correct.  This  is  a 
rather  safe  conclusion,  but  nevertheless  an  important 
one,  inasmuch  as  they  are  both  thus  stamped  with 
the  partial  approval  of  scientists  highly  familiar  with 
the  subject. 

50.  Ladd- Franklin  Theory.  —  In  this  theory  4  the 
rods  and  cones  are  used.  Colorless  sensations  white, 
gray,  and  black,  are  assumed  to  be  caused  by  a  primi- 
tive photo-chemical  substance  which  is  composed  of 
many  'gray'  molecules.  These  exist  in  their  primi- 
tive state  only  in  the  rods,  but  upon  dissociation 
they  cause  the  colorless  sensation.  In  the  cones 
the  gray  molecules  undergo  development  and  for 
some  reason  only  a  portion  of  the  molecule  becomes 
dissociated  by  rays  of  a  given  wave-length  or  color. 
The  evolution  of  the  gray  molecule  is  assumed  to  take 
place  in  three  stages  diagrammatically  shown  in  Fig. 
87.  In  the  first  stage  the  gray  molecule  exists,  but 
is  so  constructed  that  it  is  disintegrated  by  light  of 


THEORIES   OF  COLOR   VISION 


187 


STAGE  1 


STAGE:  z 


all  colors,  thus  producing  a  white  or  a  gray  sensation. 
In  the  second  stage  .  the  molecule  has  become  more 
complex  and  contains  two  groupings.  The  disso- 
ciation of  one  of  the  latter 
causes  a  yellow  sensation 
and  the  other,  blue.  Their 
simultaneous  dissociation 
causes  a  sensation  of  white 
or  gray.  Molecules  are 
assumed  to  exist  in  this 
stage  in  the  outer  zone  of 
the  retina,  where  neither 
red  nor  green  can  be  per- 
ceived as  such.  In  the 
third  stage  the  yellow 
grouping  is  divided  into 
two  new  combinations,  the 
dissociation  of  one  giving 
rise  to  a  red  sensation,  the 
other  producing  a  green 
sensation.  If  the  red  and 
green  are  dissociated  sim- 

n1tfltipniiQlv  vpllnw  Qpn  Fig.  87.  —  The  evolution  of  the  Ladd- 
UltaneOUSiy,  yeilOW  Sen-  Franklin  gray  molecule. 

sation    results,    while    all 

three  (red,  green,  and  blue)  together  produce  gray. 
There  is  much  of  interest  in  this  theory,  and  it 
appears  to  explain  many  observed  facts  satisfactorily. 

51.  Eldridge-  Green  Theory. — Boll  discovered  a 
substance  diffused  in  the  retina  which  has  been 
named  visual  purple.  This  discovery  gave  rise  to 
hopes  that  a  photochemical  theory  of  vision  would 
explain  the  observed  facts,  inasmuch  as  the  visual 
purple  was  found  to  be  sensitive  to  light.  However, 
after  the  elaborate  work  of  Kiihne  the  visual  purple 
lost  much  of  its  significance  in  this  respect.  If  an 


188  COLOR  AND   ITS  APPLICATIONS 

eye  which  has  been  unexposed  to  light  for  some  time 
be  cut  out  in  a  room  illuminated  by  means  of  a  dim 
red  light,  on  removing  the  retina  it  appears  a  purple 
color  under  ordinary  light.  The  color  fades  rapidly 
on  exposure  to  ordinary  intensities  of  illumination, 
passing  through  red  and  orange  to  yellow,  finally 
disappearing.  The  yellow  appearance  is  supposed 
to  be  due  to  the  formation  of  another  pigment,  the 
visual  yellow.  The  appearance  of  the  preceding 
stages  is  thought  to  be  due  to  mixtures  of  the  visual 
purple  and  visual  yellow  in  various  proportions.  It 
apparently  has  been  established  that  normally  the 
visual  purple  is  confined  to  the  outer  portions  of  the 
rods.  It  is  extracted  readily  by  a  watery  solution  of 
bile  salts.  Spectroscopic  examination  of  this  solu- 
tion shows  it  to  have  a  maximum  absorption  for 
yellow-green  rays  and  a  minimum  for  red  rays  and 
is  bleached  by  the  rays  in  about  the  proportion  that 
it  absorbs  them.  The  visual  purple  is  so  sensitive 
to  light  that  pictures  of  very  bright  objects  have  been 
seen  in  purple  and  white  on  retinae  of  the  eyes  of 
animals.  Such  experiments  have  been  performed 
by  exposing  an  eye  extracted  from  an  animal  which 
has  been  kept  in  darkness  for  some  time. 

Many  attempts  have  been  made  to  weave  the 
visual  purple  into  a  theory  of  vision.  Edridge-Green  5 
has  done  so,  as  briefly  outlined  below.  He  assumes 
'that  the  cones  of  the  retina  are  insensitive  to  light, 
but  sensitive  to  the  changes  in  the  visual  purple. 
Light  falling  upon  the  retina  liberates  the  visual 
purple  from  the  rods,  and  it  is  diffused  into  the  fovea 
and  other  parts  of  the  rod  and  cone  layer  of  the 
retina.  The  decomposition  of  the  visual  purple  by 
light  chemically  stimulates  the  ends  of  the  cones 
(probably  through  the  electricity  which  is  produced) 


THEORIES   OF  COLOR  VISION  189 

and  a  visual  impulse  is  set  up  which  is  conveyed 
through  the  optic  nerve  to  the  brain.'  He  further 
assumes  that  'the  visual  impulses  caused  by  the 
different  rays  of  light  differ  in  character  just  as 
the  rays  of  light  differ  in  wave-length.  Then  in  the 
impulse  itself  we  have  the  physiological  basis  of  the 
sensation  of  light,  and  in  the  quality  of  the  impulse 
the  physiological  basis  of  the  sensation  of  color.' 
He  also  assumes  'that  the  quality  of  the  impulse  is 
perceived  by  a  special  perceptive  center  in  the  brain 
within  the  power  of  perceiving  differences  possessed 
by  that  center  or  portion  of  that  center.  According 
to  this  view  the  rods  are  not  concerned  with  trans- 
mitting visual  impulses,  but  only  with  the  visual 
purple  and  its  diffusion.'  On  this  theory  he  attempts 
to  explain  all  the  observed  facts  encountered.  He 
concludes  the  paper,  from  which  the  foregoing  is 
quoted,  by  stating  that  'I  am  not  aware  of  any  fact 
which  does  not  support  the  theory.'  Needless  to 
say,  however,  there  are  those  who  entertain  a  dif- 
ferent opinion  on  this  last  and  other  points. 


REFERENCES 

1.  Handbuch  der  Physiologischen  Optik. 

2.  Grundziige  der  Lehre  vom  Lichtsinn,  Leipzig,  1905,  p.  41. 

3.  Archif.  f.  Opth.  1881,  I,  p.  55;   1884,  I,  p.  15. 

4.  Zeit.  f.  Psych,  u.  Physiol.  der  Sinnesorgane,  1892. 

5.  Lancet,  Oct.  2, 1909. 

OTHER    REFERENCES 

Ebbinghaus,  Theorie  des  Farbensehens,  Zeit.  f.  Psych,  u.  Physiol, 
der  Sinnesorgane,  1893. 

A.  Koenig,  Ges.  Abhandlungen,  Leipzig,  1903. 

M.  Greenwood,  Jr.,  Physiology  of  the  Special  Senses. 

W.  Nagel,  Handbuch  der  Physiologic  des  Menschen,  1905. 


190  COLOR  AND   ITS  APPLICATIONS 

W.  Wtindt,  Grundzuge  der  Physiologischen  Psychologic,  Leipzig, 
1911. 

H.  Aubert,  Grundzuge  der  Physiologischen  Optik,  Leipzig,  1876. 

Captain  W.  de  W.  Abney,  Colour  Vision,  London,  1895. 

F.  W.  Edridge-Green,  Colour  Blindness  and  Colour  Perception, 
London,  1909. 

W.  Nicati,  Physiologic  Oculaire,  Paris,  1909. 

J.  H.  Parsons,  Colour  Vision,  New  York,  1915. 


CHAPTER   IX 
COLOR  PHOTOMETRY 

52.  The  relation  between  radiation  of  various 
wave-lengths  and  luminous  sensation  has  long  been 
the  subject  of  investigation;  but,  notwithstanding  the 
extensive  data  obtained,  there  is  no  general  agree- 
ment as  to  a  method  that  yields  correct  results. 
Much  of  the  early  data  is  practically  useless  at  the 
present  time,  owing  to  the  lack  of  control  of  various 
influential  factors,  due  to  the  absence  of  definite 
knowledge  regarding  their  ability  to  influence  the 
judgment  of  brightness.  This  data  of  course  has 
served  well  in  lighting  the  pathway  of  investigation. 

From  foregoing  chapters  it  has  been  seen  that  the 
size  of  the  photometric  field,  owing  to  the  variation 
of  retinal  sensibility  to  colored  light,  is  of  importance 
in  color  photometry.  Due  to  the  Purkinje  phenom- 
enon the  brightness  at  which  measurements  are  made 
also  affects  the  results.  In  this  connection  it  should 
be  noted  that  the  brightness  of  the  photometric  field 
as  seen  by  the  eye  is  sometimes  greatly  reduced  by 
absorption  of  light  in  the  optical  path  and  by  a  small 
ocular  aperture  or  artificial  pupil  of  the  instrument. 
Other  factors,  such  as  the  adaptation  of  the  eye  and 
the  character  of  the  surrounding  field,  are  influential. 
Most  important  is  the  method,  for  no  two  methods 
yield  exactly  the  same  results.  It  is  well  to  remem- 
ber that  the  brightness  of  a  colored  area  is  so  influ- 
enced by  its  environment  that  its  determination,  in 
comparison  with  a  standard  in  an  isolated  photometric 

191 


192  COLOR  AND  ITS  APPLICATIONS 

field,  is  not  in  general  a  measure  of  its  brightness 
as  it  appears  in  another  environment.  Therefore 
the  photometry  of  colored  surfaces  yields  measure- 
ments of  the  brightness  in  terms  of  the  particular 
standard  used  and  the  results  depend  upon  the  hue, 
saturation,  and  brightness  of  the  comparison  field, 
the  surroundings,  the  condition  of  the  eye,  and  the 
photometric  method  used. 

53.  Primary  Methods  of  Photometry.  — A  method 
of  photometry  should  ordinarily  have  for  its  object 
the  measurement  of  the  illuminating  value  of  the 
illuminant  with  respect  to  its  ability  to  make  objects 
visible  by  reflected  or  transmitted  light.  The  method 
of  visual  acuity  has  been  proposed  and  used  by  some 
for  the  measurement  of  illumination.  Obviously  such 
a  method  determines  the  defining  power  of  the  illumi- 
nant or  of  the  light  reflected  or  transmitted  by  an 
object.  The  criterion  of  such  a  method  is  usually 
the  discrimination  of  fine  detail  or  the  adjustment  of 
the  illumination  so  that  the  detail  appears  to  be 
equally  legible  as  compared  with  a  standard.  In 
general  this  method  is  quite  insensitive,  and  the 
results  are  greatly  dependent  upon  fatigue  and  the 
state  of  adaptation  of  the  eye.  However,  there  is 
another  complication,  that  of  the  spectral  character 
of  light.  The  experiments  of  the  author  described 
in  #37  showed  that  a  reduction  in  the  amount  of 
illumination  or  brightness  of  the  acuity  test  object, 
when  accompanied  by  certain  changes  in  the  spectral 
character  of  the  illuminant,  sometimes  results  in  an 
increase  in  visual  acuity.  From  these  experiments 
it  is  seen  that  the  method  of  visual  acuity  cannot 
be  depended  upon  to  determine  the  relative  illumi- 
nating values  of  illuminants  or  of  lights  altered  in 
spectral  character  by  reflection  from  colored  surfaces. 


COLOR  PHOTOMETRY  193 

The  visual  acuity  method  is  valuable  in  many  cases, 
but  it  must  be  understood  that  a  large  amount  of  our 
seeing  does  not  include  the  perception  of  fine  detail 
at  the  limits  of  discrimination,  but  only  requires  the 
recognition  of  relatively  large  surfaces  through  dif- 
ferences in  color  and  brightness.  Even  reading  under 
ordinary  conditions  does  not  involve  visual  acuity  at 
the  limit  of  discrimination,  for  the  illumination  is 
usually  far  above  that  necessary  to  distinguish  the 
type,  and  it  has  been  shown  that  in  reading  the  eyes 
recognize  characters  in  groups,  travel  by  jumps,  and 
come  to  rest  only  a  few  times  during  their  progress 
across  a  page. 

At  one  time  the  critical  frequency  method  was  ,/ 
looked  upon  as  a  possible  solution  of  the  problem  of 
color  photometry.  It  was  shown  in  #  38  (Figs.  75  and 
76)  that  if  a  brightness  be  alternated  against  darkness 
there  is  a  certain  minimum  frequency  of  alternation, 
called  the  critical  frequency,  at  which  flicker  just 
disappears.  In  general,  it  has  been  found  that  the 
critical  frequency  varies  directly  as  the  logarithm 
of  the  illumination  or  brightness  of  the  test  surface. 
Thus  plotting  these  two  factors  yields  a  straight  line, 
the  slope  of  which  is  different  for  lights  of  different 
colors.  The  slope  of  this  straight  line  changes 
abruptly  at  a  very  low  illumination  (thought  by  some 
to  be  the  point  at  which  the  cones  just  cease  to  be 
sensitive  to  light)  for  lights  of  all  colors  with  the  ex- 
/ yteption  of  red.  By  this  method  two  surfaces  were 
W  assumed  to  be  equally  bright  when  their  critical 
or  vanishing-flicker  frequencies  were  equal.  The 
method  has  proved  too  insensitive  for  practical  use 
and  too  susceptible  to  various  physiological  factors, 
such  as  fatigue  and  adaptation. 

The    ordinary    direct    comparison    or    equality-of- 


194  COLOR  AND   ITS  APPLICATIONS 

brightness  method  is  claimed  by  many  to  involve  the 
only  true  criterion  for  the  measurement  of  brightness. 
Others  claim  that  its  shortcomings  have  disqualified 
it  for  use  in  the  photometry  of  lights  of  different  colors 
and  have  accepted  the  flicker  method.  The  latter 
method,  which  is  in  high  favor  for  color  photometry, 
has  not  been  proved  to  measure  the  true  brightness 
of  a  colored  surface  —  if  there  be  such  a  thing.  Nev- 
ertheless, the  uncertainties  in  the  measurements  by 
the  direct  comparison  method  has  brought  this  or- 
dinary method  into  disfavor  with  many  photometri- 
cians  for  the  photometry  of  lights  differing  in  color. 
The  flicker  method  involves  the  alternation  of  two 
brightnesses  —  the  standard  and  the  unknown.  A 
match  is  made  with  this  instrument  by  altering  both 
the  brightness  of  the  field  due  to  one  of  the  sources 
and  the  frequency  of  alternation.  The  two  bright- 
,  nesses  are  considered  equal  when  the  frequency  of 
alternation  is  such  that  a  slight  change  of  either 
illumination  produces  a  just  perceptible  flicker.  When 
there  is  no  color  difference  between  the  two  bright- 
nesses being  compared,  the  theoretical  frequency 
should  be  zero,  but  owing  to  imperfections  in  the 
photometric  apparatus  this  condition  is  never  obtained. 
The  flicker  photometer  is  based  upon  the  fact  that 
color  difference  is  eliminated  by  mixing  the  two 
brightnesses  by  persistence  of  vision,  the  color  flicker 
apparently  disappearing  before  the  brightness  flicker. 
Numerous  instruments  have  been  devised  for  this 
purpose,  but  all  involve  this  fundamental  principle. 
No  extended  comparative  study  of  flicker  photometers 
has  been  made,  although  it  is  possible  that  instru- 
ments differing  in  design  might  yield  different  results. 
For  instance,  in  many  instruments  the  stimulus 
changes  abruptly  from  one  color  to  another,  but  in 


COLOR  PHOTOMETRY 


195 


some  the  stimuli  dissolve  into  each  other.  Whether 
or  not  such  instruments  yield  different  results  is  a 
question  to  be  solved  by  further  investigation. 

To  summarize,  the  methods  of  visual  acuity  and 
critical    frequency    are    impracticable    owing    to    their 


10 


» 


y 


\ 


\ 


N>.-^3 


50       0.5Z      0.54       0.56      05&       0.60       0.62       0.64       0.66 


Fig.  88.  —  The  results  of  four  methods  of  photometry  (Ives). 

extreme  insensitiveness  and  the  influence  of  eye  fa- 
tigue and  adaptation.  The  influence  of  the  spectral 
character  of  light  further  complicates  the  visual 
acuity  method.  The  direct  comparison  method,  though 
claimed  by  many  to  yield  measurements  of  'true' 
brightness  is  unpopular,  owing  to  the  uncertainties 
in  the  measurements.  The  flicker  method,  however, 
owing  to  its  elimination  of  color  difference  and  high 


196  COLOR  AND   ITS  APPLICATIONS 

sensibility,  had  won  many  ardent  supporters  even 
before  extensive  investigations  of  the  method  had 
been  made. 

In  Fig.  88  are  shown  data  obtained  by  Ives  x 
with  the  four  methods.  In  each  case  the  standard 
was  the  total  light  from  a  tungsten  lamp.  Spectral 
colors  were  compared  with  this  white  standard. 
Curve  V  was  obtained  by  the  visual  acuity  method; 
D,  by  the  direct  comparison;  F,  by  the  flicker;  and 
C,  by  the  critical  frequency  method.  If  the  four 
methods  gave  identical  results,  the  curves  would  coin- 
cide. The  general  shapes  and  positions  of  the  max- 
ima are  similar,  but  the  areas  under  the  curves  are 
very  different.  The  enormous  area  under  curve  V 
is  in  accord  with  the  previous  work  of  Bell2  and  of 
Luckiesh,3  which  showed  that  acuity  was  much 
better  in  monochromatic  light  than  in  light  of  ex- 
tended spectral  character. 

54.  Secondary  Methods  of  Color  Photometry.  - 
Various  schemes  have  been  proposed  and  developed 
for  eliminating  color  difference  in  heterochromatic 
photometry,  such  as  the  use  of  colored  filters,  and 
physical  and  chemical  photometers  used  with  filters 
that  properly  weigh  the  energy  of  various  wave- 
lengths according  to  their  light-producing  effects. 
Among  the  latter  possibilities  are  the  radiometer, 
thermopile,  selenium  cell,  photo-electric  cell,  and 
photographic  plate.  Obviously,  in  order  to  reduce 
measurements  to  absolute  values,  the  transmission 
coefficients  of  the  colored  filters  must  be  determined 
by  some  acceptable  method.  Likewise,  determina- 
tions of  the  relation  between  radiation  of  various 
wave-lengths  and  the  corresponding  luminous  sensa- 
tions and  of  the  sensibility  of  the  instruments  to 
energy  of  various  wave-lengths  must  be  made  before 


COLOR  PHOTOMETRY  197 

the  results  obtained  with  the  selenium  cell,  the  radiom- 
eter, filters,  etc.,  are  useful  in  measuring  brightness. 

Crova 4  suggested  as  a  method  of  comparing  lights 
possessing  continuous  spectra,  but  differing  in  color, 
the  determination  of  their  intensities  at  one  wave- 
length, 0.582/z.  The  lights  to  be  compared  in  this 
manner  must  not  differ  much  in  spectral  energy  dis- 
tribution from  the  black  body.  Rayleigh,5  Nernst,6 
Fery  and  Cheneveau,7  Lucas,8  Rasch,9  and  others 
have  made  various  applications  and  modification  of 
Crova's  original  proposal.  The  filter  used  by  Crova 
consisted  of  an  aqueous  solution  of  anhydrous  ferric 
chloride  (22.321  grams)  and  crystallized  nickelous 
chloride  (27.191  grams),  the  total  volume  being  100 
c.c.  at  15°  C.  A  thickness  of  7  mm.  of  this  solu- 
tion was  used  which  transmits  energy  from  0.63^- to 
0.534,u  with  a  maximum  of  'transmission  at  0.582/*,  the 
wave-length  which  Crova  found  to  be  satisfactory  for 
carrying  out  his  proposed  scheme.  Ives  10  tested 
Crova's  method  by  comparing  the  luminous  intensities 
of  a  tantalum  and  a  carbon  incandescent  lamp  at 
various  wave-lengths.  He  found  that  the  wave-length 
for  such  a  comparison  lies  between  0.56^  and  0.58/4, 
depending  on  the  range  of  temperature.  The  latter 
wave-length  was  found  to  hold  best  of  all  within  the 
limits  of  temperature  represented  by  ordinary  incan- 
descent lamps  of  that  time.  Twelve  years  ago 
Fabry  n  recommended  the  use  of  two  or  more  col- 
ored solutions  for  eliminating  color  difference,  having 
first  calibrated  these  solutions  for  thickness  and  trans- 
mission by  an  acceptable  method.  Aniline  dyes  were 
not  used,  because  of  the  need  for  definite  and  re- 
producible solutions.  By  using  two  solutions,  A  and 
Bj  he  was  able  to  match  the  Carcel  lamp  with  almost 
any  illuminant.  The  solutions  were  made  as  follows: 


198  COLOR  AND   ITS  APPLICATIONS 

A.  Crystallized  copper  sulphate 1  gram 

Commercial  ammonia  (density  0.93) 100  c.  c. 

Water  sufficient  to  make  one  liter. 

B.  Potassium  iodide 3  grams 

Iodine 1  gram 

Water  sufficient  to  make  one  liter. 

Ives  and  Kingsbury  12  have  recently  investigated 
the  problem  of  obtaining  suitable  solutions  that  would 
eliminate  color  difference  after  the  manner  pro- 
posed by  Fabry.  They  developed  a  yellow  solution 
containing  100  grams  of  cobalt  ammonium  sulphate, 
0.733  grams  of  potassium  dichromate,  10  c.c.  of 
1.05  sp.  gr.  nitric  acid,  and  distilled  water  to  make 
one  liter  at  20  deg.  centigrade.  The  method  of 
preparation  is  considered  very  important  and  is  pre- 
sented in  detail  in  the  original  paper.  Of  course  a 
given  depth  or  concentration  of  the  solution  has  a 
different  transmission  for  illuminants  of  different 
spectral  character.  The  transmission  values  were 
determined  by  means  of  a  flicker  photometer  by 
averaging  the  results  obtained  by  specially  selected 
observers.  The  transmission  of  the  solution  was 
found  to  vary  considerably  for  different  temperatures 
and  the  character  and  cleanliness  of  the  glass  sides 
of  the  containing  cell  were  found  to  be  of  consid- 
erable importance.  It  was  found  possible  to  eliminate 
color  difference  in  comparing  many  illuminants  with 
the  carbon  lamp  standard  by  placing  the  solution  on 
either  one  side  or  the  other  of  the  photometer. 

Many  have  used  aniline  dyes  and  colored  glasses. 
In  practical  photometry  the  use  of  colored  glasses 
appears  to  be  satisfactory  for  a  large  amount  of  work. 
The  carbon  lamp  operating  at  about  4  w.p.m.h.c.  is 
the  present  standard  of  luminous  intensity.  Properly 


COLOR  PHOTOMETRY  199 

tinted  bluish  glasses  used  with  this  standard  will 
eliminate  the  color  difference  when  comparing  tung- 
sten lamps  with  it.  The  transmissions  of  the  tinted 
glasses  can  be  obtained  by  averaging  the  determina- 
tions of  a  large  number  of  observers,  using  the  direct 
comparison  method.  Such  a  procedure  is  being  used 
successfully  in  several  laboratories  for  the  above 
work  where  the  color  difference  is  not  excessive. 
However,  it  is  not  a  solution  of  the  general  problem 
of  color  photometry. 

Houston  13  in  1911  proposed  the  use  of  a  filter 
composed  of  two  solutions  —  copper  sulphate  and 
potassium  dichromate  —  for  closely  approximating  in 
transmission  the  luminosity  curve  of  the  eye,  this 
filter  to  be  used  with  an  energy-measuring  instru- 
ment. Koenig's  visibility  data  were  used  as  a  basis 
for  developing  the  solution.  A  proper  solution  would 
transmit  rays  of  various  wave-lengths  in  the  propor- 
tions corresponding  to  the  relative  light-producing 
values  of  the  various  rays.  It  is  necessary  to  cut 
off  both  the  infra-red  and  ultra-violet  rays  and  to  re- 
duce the  visible  rays  in  just  the  correct  relative  pro- 
portions so  that  an  energy-measuring  instrument 
(bolometer,  thermopile,  or  radiometer)  will  record 
data  proportional  to  the  luminous  intensity.  A  dis- 
advantage of  such  instruments  is  found  in  their 
extreme  sensitiveness  to  outside  disturbances.  For 
instance,  the  galvanometer  used  in  the  procedure 
must  be  of  a  high  order  of  sensibility  and  therefore 
must  be  set  up  where  it  will  be  free  from  mechanical 
and  magnetic  disturbances.  Karrer,14  recently  follow- 
ing Houston's  lead,  similarly  employed  the  visibility 
data  obtained  by  Ives.  By  using  three  solutions  he 
was  able  to  produce  a  screen  whose  transmission 
curve  closely  approached  this  luminosity  curve  of  the 


200 


COLOR  AND   ITS  APPLICATIONS 


eye.  The  solutions  were  made  by  dissolving  (1) 
57.519  grams  of  cupric  chloride,  (2)  1.219  grams  of 
potassium  bichromate,  and  (3)  9.220  grams  of  ferric 
chloride,  each  in  one  liter  of  water.  A  triple  cell 
was  used,  each  compartment  being  1  cm.  thick. 

The  selenium  cell  has  been  used  for  stellar  pho- 
tometry and  for  other  special  work,  owing  to  its 
change  in  resistence  on  being  illuminated.  However, 
it  has  not  yet  found  a  place  in  color  photometry,  be- 
cause at  present  it  is  too  erratic  and  undependable. 


Photo- Electric 


040 


0.45- 


Selenium 


0.50 


0.55 


0.60 


0.65 


0.70 


0.75 


V 


Fig.  89.  —  Spectral  sensibilities  of  selenium  and  photo-electric  cells  compared 
with  the  spectral  sensibility  of  the  eye. 

Its  sensibility  to  energy  of  various  wave-lengths  ap- 
pears to  depend  upon  the  method  of  making  the  cell, 
and  is  in  general  far  different  from  that  of  the  eye. 
A  sensibility  curve  is  shown  in  Fig.  89,  compared 
with  the  luminosity  curve  of  the  eye.  The  maximum 
change  in  resistance  is  usually  due  to  energy  of  the 
longer  visible  wave-lengths.  Obviously  a  filter  that 
properly  weighs  the  energy  of  various  wave-lengths 
according  to  its  light  value  and  to  the  spectral  sen- 
sibility of  the  cell,  must  be  used  for  the  photometry 
of  illuminants  of  extended  spectral  character. 

The  photo-electric  cell  has  been  used  in  special 
cases    of    scientific    investigation    for    detecting    the 


COLOR  PHOTOMETRY  201 

presence  of  radiant  energy.  Surfaces  of  potassium, 
zinc,  and  other  elements  and  compounds  in  vacuo 
exhibit  the  property  of  emitting  electrons  when  illumi- 
nated. The  maximum  effect  is  usually  found  in  the 
short-wave  visible  region,  as  illustrated  by  a  sensi- 
bility curve  of  a  photo-electric  cell,  shown  in  Fig.  89. 
As  in  the  case  of  the  selenium  cell,  the  photo-electric 
cell  is  too  erratic  at  the  present  time  to  be  adopted 
as  a  means  of  photometering  lights  of  different  colors. 
The  strengths  of  the  electronic  currents  measured 
by  means  of  a  sensitive  electrometer  or  galvanometer 
afford  a  measure  of  the  relative  intensities  of  the 
illumination  of  a  given  spectral  character  when  the 
characteristics  of  the  cell  are  shown;  that  is,  when 
the  relation  between  the  intensity  of  illumination  and 
the  photo-electric  effect  is  known.  Lights  differing 
in  spectral  character  cannot  be  compared  by  means 
of  the  photo-electric  cell  unless  a  correcting  filter  is 
used  after  the  manner  necessary  with  the  selenium 
cell. 

The  photographic  plate  affords  another  possible 
method  for  the  photometry  of  lights  of  different  color, 
but  its  general  adoption  is  discouraged,  owing  to  lack 
of  uniformity  of  the  emulsion  both  as  to  thickness 
and  sensibility.  Some  of  the  difficulty  could  be 
obviated  by  using  plates  made  of  plate  glass.  The 
panchromatic  plates  must  be  used,  because  the  ordi- 
nary plate  is  not  appreciably  sensitive  to  rays  of 
longer  wave-length  than  0.48ju,  the  maximum  of 
sensibility  being  in  the  extreme  violet  region  of  the 
spectrum.  The  relative  sensibility  of  a  certain  com- 
mercial panchromatic  plate,  for  equal  amounts  of 
energy  of  various  wave-lengths,  is  shown  in  Fig.  90 
compared  with  the  spectral  sensibility  of  the  eye. 
In  order  to  make  the  plate  record  the  values  of  col- 


202 


COLOR  AND  ITS  APPLICATIONS 


ored  brightnesses  as  determined  with  a  flicker  pho- 
tometer, an  accurate  filter  was  made  which  consisted 
of  aesculine,  tartrazine,  rhodamine,  naphthal  green, 
and  glass  three-eighths  of  an  inch  thick.  How  nearly 
this  filter  performs  its  intended  purpose  is  shown  in 
Fig.  91  by  the  circles  in  comparison  with  the  lumi- 
nosity curve  of  the  eye  which  is  represented  by  the 
full  line  curve.  This  filter  was  used  with  the  pan- 


044          0.4&  0.5Z         0.56          0.60          0.64  0.66 

Ms,  WAVE  LENGTH 

Fig.  90.  —  Spectral  sensibility  of  a  panchromatic  photographic  plate. 

chromatic  plate  considered  above,  for  which  it  was 
made  by  Ives  and  Luckiesh15  for  various  photometric 
problems.  In  using  the  photographic  plate  for  pho- 
tometric purposes  it  must  be  remembered  that,  in 
general,  the  product  of  intensity  of  illumination  and 
time  of  exposure  is  not  a  constant  for  equal  photo- 
graphic effect.  The  relation  between  exposure  and 
intensity  of  illumination  for  a  constant  photographic 
effect  as  discovered  by  Schwartzchild  is  It*  =  i  Tp 
where  I  and  i  are  the  larger  and  smaller  intensities 
and  T  and  t  are  the  larger  and  smaller  periods  of 


COLOR  PHOTOMETRY 


203 


exposure.  The  value  of  p  varies  with  different  plates, 
generally  lying  between  0.75  and  unity.  The  manner 
of  development,  the  temperature,  and  other  obvious 
factors  influence  the  results  so  that  the  photographic 
method  becomes  unattractive  except  for  special  prob- 
lems. As  already  stated,  the  use  of  these  so-called 
physical  or  chemical  photometers,  while  obviating 
color  difference  in  practise,  does  not  preclude  the 
necessity  of  establishing  the  relation  between  lumi- 
nous sensation  and  radiation  of  various  wave-lengths 
by  an  acceptable  method  of  color  photometry. 


RELATIVE  BRIGHTNESS 

—  W  tf> 

/• 

—  •». 

f 

- 

7 

\ 

/ 

\ 

/ 

v 

s 

? 

/ 

X 

x, 

*^ 

^^ 

>< 

s. 

0.40       0.44 


048 


0.52        0.56 


0.60 


0.64        0.68         0.72 


Fig.  91.  —  An  accurate  color  filter  for  the  panchromatic  plate  considered  in  Fig.  90. 

55.  Direct  Comparison  and  Flicker  Methods.  - 
Only  two  primary  methods  for  the  photometry  of 
lights  differing  in  color  are  worthy  of  consideration, 
namely  the  direct  comparison  and  flicker  methods, 
the  other  two  being  ruled  out  of  consideration  for 
reasons  already  given.  These  two  methods  have 
been  compared  by  many  observers,  but  much  of  the 
work  is  so  incomplete  that  it  yields  little  data  for  a 
thorough  comparison.  It  is  desirable  that  the  method 
finally  acceptable  for  photometering  lights  of  different 
colors  should  measure  light-value  with  the  same 
order  of  definiteness  as  other  physical  measurements 
are  obtained. 

Dow16    compared    these    two    methods    by    using 


204  COLOR  AND   ITS  APPLICATIONS 

colored  lights  produced  by  means  of  red  and  green 
glasses  at  different  intensities  and  with  different 
field  sizes.  He  found  with  the  direct  comparison 
method  that  the  ratio  of  the  red  to  the  green  bright- 
ness decreased  with  decreasing  illumination,  the 
decrease  being  rapid  below  0.3  meter  candle,  —  the 
well-known  Purkinje  phenomenon.  With  the  flicker 
method  this  decrease  was  slight.  With  a  small  pho- 
tometer field  the  change  in  the  ratio  of  red  to  green 
was  considerably  less.  In  general  Dow's  results 
indicate  that  the  flicker  method  is  less  influenced  by 
the  size  of  the  field  or  by  a  change  in  the  illumina- 
tion than  the  direct  comparison  method. 

P.  S.  Millar  17  compared  mercury  vapor  arcs  with 
incandescent  lamps  over  a  wide  range  of  illumina- 
tions. The  Purkinje  phenomenon  was  in  evidence 
in  the  direct  comparison  measurements  but  absent 
in  the  results  obtained  with  a  flicker  photometer. 
In  other  words,  with  the  former  method  the  apparent 
brightness  of  the  side  of  the  photometer  field  illumi- 
nated by  light  from  the  mercury  arc  did  not  decrease 
as  rapidly  as  the  brightness  of  the  other  side  illumi- 
nated by  light  from  an  incandescent  lamp,  as  the 
illumination  decreased.  Stuhr 18  compared  the  four 
methods — namely,  visual  acuity,  critical  frequency, 
direct  comparison,  and  flicker.  He  found  the  critical 
frequency  and  flicker  methods  to  yield  identical 
results,  but  these  differed  from  the  results  by  the 
other  two  methods.  Various  physiological  factors,  such 
as  field  size  and  illumination,  were  not  considered. 

Ives  19  carried  out  an  extensive  series  of  investi- 
gations which  represent  the  most  elaborate  and 
thorough  work  yet  done  on  the  problem.  He  con- 
cluded that  the  flicker  method  is  more  sensitive  than 
the  direct  comparison  method  and  that  the  results 


COLOR  PHOTOMETRY  205 

are  more  reproducible.  He  discovered  that  the  flicker 
method  exhibited  a  'reversed  Purkinje  effect'  and 
found,  as  other  investigators  had,  that  the  two 
methods  yielded  different  results  in  general,  but  con- 
cludes that  the  flicker  method  yields,  under  certain 
specified  conditions,  a  measure  of  true  brightness. 
Much  evidence  obtained  throughout  these  investiga- 
tions and  some  obtained  by  the  author  and  others 
point  favorably  to  the  flicker  method  as  the  best 
method  of  photometry.  However,  notwithstanding 
the  extensive  investigations,  some  take  the  stand  that 
the  case  has  neither  been  decided  against  the  direct 
comparison  method  nor  in  favor  of  the  flicker  method. 
This  conclusion  is  perhaps  justifiable.  However, 
considering  the  unsatisfactoriness  of  the  former 
method,  there  is  considerable  virtue  in  the  adoption 
of  the  latter  method  with  its  many  satisfactory  fea- 
tures in  default  of  a  method  which  has  been  definitely 
proved  to  yield  the  desired  measurements.  Ives  in 
his  early  papers  did  not  emphasize  the  differences 
in  the  results  obtained  by  the  two  methods.  His 
results  were  plotted  in  the  form  of  luminosity  curves 
of  the  eye,  so  that  without  careful  inspection,  the 
results  by  the  two  methods,  under  certain  conditions 
of  high  illumination  and  small  field  size,  do  not 
appear  to  differ  greatly.  In  order  to  determine  the 
magnitude  of  these  outstanding  differences  the  au- 
thor 20  carried  out  an  investigation,  a  portion  of  the 
results  (L)  being  plotted  in  Fig.  92.  Red  and  blue- 
green  lights  were  used.  The  ratio  of  the  intensity 
of  the  red  to  that  of  the  blue-green  light  is  plotted 
for  a  wide  range  of  illuminations.  The  illumination 
values  are  those  obtained  with  the  flicker  photometer 
and  a  standard  tungsten  lamp,  but  are  not  corrected 
for  the  absorption  of  the  photometer,  which,  owing  to 


206 


COLOR  AND   ITS  APPLICATIONS 


a  complex  optical  path,  was  considerable,  or  for 
reduction  due  to  the  small  artificial  pupil.  It  is  seen 
that  the  flicker  method  exhibits  a  reversed  Purkinje 
effect  and  the  direct  comparison  method  the  true 
Purkinje  effect,  and  further  that  the  ratio  of  the 
red  to  the  blue-green  brightness  obtained  by  the 
direct  comparison  method  is  only  about  62  per 
cent  of  that  obtained  by  the  flicker  method  for 


1.0 

1.4 
1.3 
1.2 
I.I 

1.0 
0.9 

0.7 
0.6 
0.5 
0.4 
0.3 
0,2 
0.1 

( 

J 

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*ER 

)  C 

hftf 

rvf 

>rf< 

1 

^<. 

•  — 

< 

v 

( 

YFL 

CKi 

F/1?)  Observer  L. 

IQU 

ALI 

TYt 

•>FB 

RIG/ 

frfi 

ESS) 

Ob 

sen 

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ser 

L 

f 

X 

—  c 

{ 

)    1    Z    3  4    5  6    1 

METER  CAttDLESCON 

8    9    10    II   12   13  14  15 
PHOTOMETER  SCREEN) 

Fig.  92. — Results  by  flicker  and  direct  comparison  photometers,  illustrating 
differences  including  the  Purkinje  effect  and  a  reversed  effect. 

a  large  range  of  illuminations.  The  results  as  to  the 
reversed  Purkinje  effect  were  verified  in  general  by 
another  observer  (K).  It  is  seen  that  he  did  not 
obtain  the  same  results  as  the  author,  even  with 
the  flicker  photometer,  by  about  13  per  cent.  A 
similar  difference,  though  in  general  not  as  great,  is 
found  in  Ives*  data  when  the  lights  of  the  correspond- 
ing dominant  hues  (0.64/i  and  0.52/z)  are  compared, 
even  though  in  his  measurements  the  spectral  colors 
were  always  balanced  against  a  white  light.  The 
same  extreme  difference  in  the  results  by  the  two 


COLOR  PHOTOMETRY  207 

methods  was  confirmed  by  the  writer  using  the  same 
glasses  a  year  later.     The  field  size  in  the  foregoing 
experiments  was  rather  large  —  about  ten  degrees  - 
but    a    large    difference    persists    even    with    smaller 
fields,  though  not  to  such  an  extent. 

Morris-Airey 21  suggested  that  the  differences 
between  the  two  methods  might  be  due  to  the  dif- 
ferent rates  of  rise  of  the  sensation  with  different 
colors.  The  author  22  studied  this  factor  and  showed 
that  the  maximum  of  a  flickering  red  light  was  con- 
siderably greater  than  that  of  the  blue-green  light 
for  a  large  range  of  flicker  frequencies  when  the 
brightnesses  of  the  two  lights  were  those  obtained 
by  a  direct  comparison  balance.  This  in  itself  did 
not  prove  which,  if  either,  is  the  correct  method. 
However,  another  experiment  was  performed  which 
is  perhaps  as  convincing  as  any  yet  performed  in 
indicating  that  the  flicker  photometer  when  properly 
used  is  inappreciably  influenced  by  the  different 
rates  of  growth  and  decay  of  color  sensations.  Lights 
differing  greatly  in  spectral  character,  but  alike  in  hue, 
were  compared  by  the  two  methods  and  identical 
results  were  obtained.  Two  yellow  lights  were  ob- 
tained by  means  of  filters  of  aqueous  solutions  of 
potassium  dichromate  used  before  a  tungsten  lamp. 
In  one  of  the  solutions  was  dissolved  some  neodym- 
ium  ammonium  nitrate,  which  absorbed  all  the  spec- 
tral yellow.  The  two  lights  now  nearly  matched  in 
hue  and  were  readily  brought  to  an  exact  color-match 
by  altering  the  concentration  and  by  adding  a  little 
ordinary  yellow  or  orange  dye  to  one  solution.  The 
spectral  characters  of  these  two  lights  are  shown  in 
c  and  dj  Fig.  17.  Two  'white'  lights  were  also  com- 
pared, one  consisting  of  the  total  light  from  a  tung- 
sten lamp,  the  other  being  made  up  of  narrow  regions 


208  COLOR  AND  ITS  APPLICATIONS 

4 

o£  the  spectrum,  respectively  in  the  red  and  blue- 
green.  In  both  cases  no  difference  in  the  ratio  of 
the  intensities  of  the  two  lights  of  the  same  color 
was  detected  in  the  results  by  the  two  methods, 
the  accuracy  being  well  within  one  per  cent.  It  was 
also  shown  that  red  and  blue-green  lights  add, 
whether  by  direct  superposition  or  by  alternately 
flickering  them  as  in  the  flicker  photometer,  when 
the  flicker  is  not  more  than  barely  apparent. 

Further  investigations  may  show  that  the  flicker 
photometer  is  influenced  by  the  different  rates  of 
growth  and  decay  of  color  sensations,  but  the  fore- 
going experiments  indicate  that  such  influence  is 
slight.  Ferree  23  has  attacked  this  problem  and  has 
reported  some  interesting  preliminary  results.  The 
flicker  method  possesses  many  desirable  character- 
istics, yet  at  present  it  can  hardly  be  accepted  as 
yielding  'true'  measurements  of  brightness  unless 
the  difference  in  the  results,  obtained  by  this  method 
and  by  the  direct  comparison  method,  be  ignored. 
Where  color  differences  are  large  —  just  where  such 
a  method  as  the  flicker  method  is  most  desired  —  the 
results  by  the  two  methods  vary  most  widely. 

56.  Luminosity  Curve  of  the  Eye. — Ives  found 
that  the  spectral  luminosity  curve  obtained  with  the  di- 
rect comparison  photometer  by  the  *  cascade'  method 
(involving  small  steps  of  slight  hue  difference)  agrees 
at  high  illumination  for  a  small  field  with  the 
curve  obtained  by  the  flicker  photometer.  He  also 
found  that  the  latter  method  fulfilled  certain  funda- 
mental axioms,  namely,  that  the  sum  of  several  indi- 
vidual brightnesses  of  different  hue  must  equal  the 
brightness  of  the  whole  and  that  if  each  of  two  bright- 
nesses of  different  hue  equal  a  third  brightness,  they 
must  be  equal  to  each  other,  while  the  direct  com- 


COLOR  PHOTOMETRY 


209 


parison  method  did  not.  While  these  experiments 
point  with  favor  to  the  flicker  method,  it  is  true  that 
a  method  can  fulfill  these  requirements  and  yet  not 
yield  measurements  of  'true'  brightness.  However, 
it  appears  at  the  present  time  that  the  balance  of 
experimental  data  is  strongly  in  favor  of  the  flicker 
photometer.  For  this  reason  the  relation  between 
radiation  of  various  wave-lengths  and  their  physi- 
ologic effect  in  producing  luminous  sensation  as 


i.o 

0.9 

0.8 

t  0.7 
to 

|  0.6 
1.0.5 

^  0.4 


1 


\ 


0.3 
0.2 


0.1 


040         0.44         045  0.5Z          056  0.60         0.64  060 

J6.  WAVELENGTH 

Fig.  93.  — Visibility  data.     (See  Table  XVI.) 

obtained  with  the  flicker  photometer  is  of  consid- 
erable interest.  Ives  24  determined  the  luminosity 
curves  of  18  observers  which  he  has  published  in 
comparison  with  the  mean  curve.  Later  he  deter- 
mined the  luminosity  curves  for  25  more  observers, 
which  mean,  he  states,  agrees  well  with  that  of  the 
previous  eighteen.  Nutting  25  has  recently  obtained 
such  data  with  21  observers.  The  apparatuses  used 
by  both  Nutting  and  Ives  were  practically  the  same 
as  shown  in  Fig.  51,  the  source  W  being  eliminated. 


210 


COLOR  AND   ITS  APPLICATIONS 


The  data  as  presented  by  Nutting  are  shown  in  Fig. 
93   and  Table    XVI   compared  with   Koenig's26  origi- 


TABLE  XVI 

The  Visibility  of  Radiation  (See 


92) 


Wave 
length  0) 

Nutting 
mean 
visibility 

Ives  mean 

Koenig  mean 

Computed 
from  Nutting's 
formula 

0.400 

0.002 

0.410 

0.003 

0.420 

0.008 

0.430 

0.012 



0.440 

0.023 

0.029  1 

0.450 

0.038 

0.047  l 

0.158 

0.460 

0.066 

0.073  i 

0.201 

0.470 

0.105 

0.107  i 

0.250 

0.480 

0.157 

0.154 

0.302 

0.135 

0.490 

0.227 

0.235 

0.370 

0.232 

0.500 

0.330 

0.363 

0.476 

0.358 

0.510 

0.477 

0.596 

0.670 

0.514 

0.520 

0.671 

0.794 

0.830 

0.675 

0.530 

0.835 

0.912 

0.950 

0.824 

0.540 

0.944 

0.977 

0.996 

0.933 

0.550 

0.995 

1.000 

0.990 

0.994 

0.560 

0.993 

0.990 

0.945 

0.993 

0.570 

0.944 

0.948 

0.875 

0.939 

0.680 

0.851 

0.875 

0.780 

0.839 

0.590 

0.735 

0.763 

0.680 

0.717 

0.600 

0.605 

0.635 

0.585 

0.585 

0.610 

0.468 

0.509 

0.492 

0.456 

0.620 

0.342 

0.387 

0.396 

0.343 

0.630 

0.247 

0.272 

0.300 

0.235 

0.640 

0.151 

0.176 

0.210 

0.158 

0.660 

0.094 

0.104 

0.128 

0.108 

0.660 

0.051 

0.068  i 

0.070 

0.072 

0.670 

0.028 

0.044  1 

0.032 



0.680 

0.012 

0.026  * 





0.690 

0.007 





0.700 

0.002 



1  Extrapolated. 

nal  data  which  was  obtained  by  the  direct  comparison 
method.  Nutting  has  extended  the  observations  well 
into  the  red  and  violet  regions  of  the  spectrum  by 


COLOR  PHOTOMETRY  211 

using  sources  emitting  line  spectra.  Koenig's  data 
are  shown  in  curve  K,  Ives*  data  in  curve  /,  and 
Nutting's  data  in  curve  N.  Nutting  developed  a 
formula  of  the  form  V=  VmR*  ea(I~R)  from  which 
the  values  given  in  Table  XVI  and  represented  in  Fig. 
91  by  the  circles  have  been  computed.  Vm  in  the 
formula  represents  the  maximum  light-producing  ef- 
fect, R  =  —*,  a  =  181,  and  V  the  visibility  or  rela- 
tive light-producing  value  of  energy  of  any  wave- 
length, A.  The  maximum  sensibility  is  at  Xmax  = 
0.555^.  The  computed  values  are  found  to  coincide 
practically  with  Nutting's  mean  luminosity  curve  be- 
tween wave-lengths,  0.48/z  and  0.65^. 

» 

REFERENCES 

1.  Phil.   Mag.   1912,  24,  p.  847. 

2.  Elec.  World,  1911,  58,  p.  637. 

3.  Elec.  World,  1911,  58,  p.  450,  p.  1252. 

4.  Comp.  Rend.  93,  p.  512. 

5.  Phil.  Mag.  June,  1885. 

6.  Phys.   Zeit.    1906,    7,  p.  380. 

7.  Bui.  Soc.  Inst.  Elec.  1909,  p.  655. 

8.  Phys.  Zeit.  1906,  6,  p.  19. 

9.  Ann.  d.  Phys.  1904,  14,  p.  193. 

10.  Phys.  Rev.  1911,  32,  p.  316. 

11.  Comp.  Rend.  Nov.  1913;  Trans.  I.  E.  S.  1913,  8,  p.  302. 

12.  Trans.  I .  E.  S.  1914,  9,  p.  795. 

13.  Proc.  Roy.  Soc.  A,  1911,  p.  275. 

14.  Lighting  Jour.  Feb.  1915;  Phys.  Rev.  1915,  5,  p.  189. 

15.  Trans.  I.  E.  S.  1912,  p.  90;  Elec.  World,  1912,  60,  p.  153. 

16.  Phil.  Mag.   1910,   19,  p.  58. 

17.  Trans.  I.  E.  S.  1909,  4,  p.  769. 

18.  Kiel,  Phil.  Diss.  Vol.  19,  1908,  p.  50. 

19.  Phil.  Mag.  1912,   24,   p.  149,  p.  170. 

20.  Elec.  World,  Mar.  1913,  p.  620. 

21.  Electrician  (Lon.),  Aug.  20, 1909,  p.  758. 

22.  Phys.  Rev.  N.  S.  1914,  4,  p.  11. 


212  COLOR  AND   ITS  APPLICATIONS 

23.  Before.  I.  E.  S.  1914. 

24.  Phil.  Mag.    1912,   24,  p.  853. 

25.  Trans.  I.  E.  S.   1914,  9,  p.  633. 

26.  Ges.  Abhandlungen. 

OTHER    REFERENCES 

On  the  Photo-electric  Cell: 

H.  Dember,  Beiblatter,  1913,  No.  16;   p.  1044. 

Nichols  and  Merritt,  Phys.  Rev.  1912,  34,  p.  475. 

F.  K.  Richtmeyer,  Trans.  I.  E.  S.  1913,  p.  459;  Phys.  Rev.  July, 

1915. 
H.  E.  Ives,  Phys.  Rev.  N.  S.  1914, 3,  p.  68,  p.  396. 

On  the  Selenium  Cell: 

Seig  and  Brown,  Phys.  Rev.  N.  S.  1914,  4,  p.  48,  p.  85;  5,  p.  65, 

p.  167. 

F.  Townsend,  Sci.  Abs.  A,  7,  2869. 
A.    H.    Pfund,    Phys.    Rev.    1912,   34,   p.   370;    Light.  Jour. 

1913,  p.  128. 

T.  Torda,  Electrician,  1906,  56,  p.  1042;   Sci.  Abs.  9,  771. 
Joel  Stebbins,  Astrophys.  Jour.  1908,  27,  p.  183. 

On  Color  Photometry: 

E.  P.  Hyde  and  W.  E.  Forsythe,  The  Visibility  of  the  Red  End 
of  the  Spectrum,  Phys.  Rev.  July,  1915 ;  Astrophys.  Jour.  Sept. 
1915. 

Irwin  J.  Priest,  A  Proposed  Method  etc.,  Phys.  Rev.  July,  1915. 

E.  F.  Kingsbury,  A  Flicker  Photometer  Attachment  for  a 
Lummer-Brodhun  Photometer,  Jour.  Frank.  Inst  Aug.  1915, 
p.  215. 

H.  E.  Ives  and  E.  F.  Kingsbury,  Flicker  Photometer  Measure- 
ments on  a  Monochromatic  Green  Solution,  Phys.  Rev.  1915, 
5,  p.  230. 


CHAPTER   X 
COLOR  PHOTOGRAPHY 

57.  At  the  present  time  no  processes  of  color 
photography  have  been  developed  which  employ  the 
simple  principle  of  fixing  the  colors  of  Nature  directly 
upon  the  photographic  plate  by  chemical  means.  O. 
Wiener1  discusses  the  use  of  body  colors  which 
would  assume  the  colors  corresponding  to  the  rays 
of  light  by  chemical  modification.  Carey  Lea2  ob- 
tained a  form  of  silver  photochloride  which  assumed 
different  colors  on  exposure  to  various  rays,  but  no 
means  was  found  for  fixing  them.  Most  of  the  com- 
mercial methods  employ  colored  media  which  repro- 
duce colors  by  one  of  the  common  methods  of  color- 
mixture.  In  the  first  place  the  emulsion  must  be 
sensitive  to  all  visible  rays,  and  preferably  the  plate 
should  be  sensitive  to  light  rays,  in  closely  the  same 
manner  as  the  eye.  There  are  no  commercial  plates 
endowed  with  the  latter  characteristic,  so  panchro- 
matic plates  are  usually  used  with  an  approximate 
color  filter.  Ray  filters  of  the  accuracy  approaching 
that  illustrated  in  Fig.  91  are  rare,  but  for  accurately 
photographing  colored  objects  in  their  true  values  of 
light  and  shade,  carefully  made  filters  must  be  used 
with  panchromatic  plates,  because  the  latter  differ 
greatly  in  spectral  sensibility  from  the  eye  (Fig.  90). 
In  other  words,  a  plate  must  be  rendered  of  the  same 
relative  sensibility  to  the  various  visible  rays  as  the 
eye  by  the  use  of  sensitizing  dyes  and  ray  filters. 

213 


214  COLOR  AND   ITS  APPLICATIONS 

Fortunately  ordinary  photography  does  not  require 
such  a  high  degree  of  accuracy. 

About  a  century  ago  Seebeck  discovered  that 
silver  chloride  becomes  tinted  by  exposure  to  light 
with  an  accompanying  chemical  action.  It  is  also 
possible  by  properly  selecting  luminescent  salts  to 
produce  a  series  of  tints  after  exposure  which  are 
very  effective.  Such  colors  cannot  be  fixed,  and 
therefore  are  of  little  practical  interest.  The  devel- 
opment of  color  photography  has  been  confined 
largely  to  two  methods.  In  one  the  phenomenon  of 
interference  of  light  waves  is  utilized  to  reproduce 
colors  directly,  while  the  other  method  is  based  upon 
the  principles  of  color-mixture -- both  additive  and 
subtractive  (#18,  #19).  In  the  latter  method  artificial 
color-screens  are  used.  Sometimes  these  are  of 
minute  size,  as  will  be  shown  later. 

58.  Lippmann  Process.  —  The  method  employ- 
ing interference  of  light  waves  is  originally  due  to 
Becquerel,3  but  Lippmann's  name  is  usually  asso- 
ciated with  the  process,  owing  to  the  improvements 
which  he  devised  after  extensive  investigation.4 
Zenker  5  in  1868  explained  the  colors  sometimes  ex- 
hibited by  spectrograms  made  on  silver  chloride 
plates  as  due  to  the  interference  of  light  waves 
reflected  from  layers  of  metallic  silver  which  are 
originally  produced  by  stationary  light  waves.  Among 
those  who  have  investigated  the  process  are  Wiener,6 
Neuhaus,7  Valenta,8  Lehmann,9  and  Ives.10 

In  the  Lippmann  process  the  sensitive  film  is 
backed  by  a  film  of  clean  mercury  which  acts  as  a 
reflector.  As  light  which  has  passed  through  the 
thin  film  strikes  the  layer  of  mercury  it  is  reflected 
back  on  its  path,  and  owing  to  the  disappearance  of 
energy  at  certain  points  through  interference,  the 


Reef 


Green 
oooooooo<  Blue 


COLOR  PHOTOGRAPHY  215 

silver  compound  is  acted  upon  only  in  layers  —  at 
the  antinodes.  The  phenomenon  is  diagrammatically 
shown  in  Fig.  94.  The  silver  compound,  instead  of 
being  acted  upon  throughout  the 
thickness  of  the  film,  is  largely 
reduced  in  thin  laminae  the  dis- 
tance between  which  is  one-half  a 
wave-length  of  the  light  producing 
them.  Especially  fine-grain  plates 

mUSt     be    USed    in    Order    tO    produce         duced  in  the  Lippmann 

the  very  minute  structure.  The 
emulsion  must  be  sensitive  to  all  colors  which 
are  so  made  by  the  use  of  certain  sensitizers. 
This  discovery  is  due  to  H.  W.  Vogel,  in  1873,  who 
found  that  silver  bromide  by  treatment  with  certain 
sensitizing  dyes,  such  as  eosine  and  cyanine,  was 
rendered  sensitive  to  rays  of  longer  wave-length  than 
when  untreated. 

If  the  plate  after  exposure  in  the  Lippmann  process 
be  developed  and  illuminated  by  white  light,  from 
various  parts  of  the  film  only  colored  light  escapes  to 
the  eye  and  a  photograph  in  colors  is  seen.  It  is 
easy  to  account  for  the  reproduction  of  pure  spectral 
colors,  but  the  general  theory  has  been  the  subject 
of  much  discussion  too  extensive  to  dwell  upon  here. 

59.  Wood  Diffraction  Process.  -  -  This  method, 
invented  by  R.  W.  Wood  in  1899,  depends  upon  the 
phenomenon  of  interference,  though  in  a  different 
manner.  It  depends  upon  the  principle  that  all  colors 
may  be  matched  in  hue  by  mixtures  of  three  primary 
colors,  red,  green,  and  blue,  each  consisting  of  a 
narrow  band  of  the  visible  spectrum.  These  spectral 
primaries  lie  near  the  regions  of  the  spectrum  cor- 
responding respectively  to  0.65ju,  0.52/z,  and  0.45/z. 
This  process  utilizes  diffraction  gratings  for  the  pro- 


216 


COLOR  AND   ITS   APPLICATIONS 


duction  of  the  primary  colors.  If  a  point  source  of 
light  or  an  illuminated  slit  be  viewed  through  a  dif- 
fraction grating  (#9),  not  only  will  an  image  of  the 
source  be  seen,  but  displaced  on  either  side  a  series 
of  spectra  will  be  seen.  The  displacement  of  the 
spectra  from  the  line  joining  the  eye  and  light  source 
will  depend  upon  the  number  of  lines  per  inch  in  the 
grating;  the  fewer  lines  per  inch  the  less  is  the  dis- 
placement. The  primary  spectral  colors  are  produced 
as  shown  in  Fig.  95.  If  a  source  of  light  S,  a  lens  L, 
and  a  grating  G,  be  arranged  as  shown,  an  image  of 
the  source  will  be  seen  on  a  screen  at  7.  With  a 


Fig.  96. — Illustrating  the  Wood  diffraction  process. 

fine  grating  a  spectrum  of  the  source  will  be  seen  on 
the  screen  extending  between  aa.  With  a  coarse 
grating  a  spectrum  of  the  source  will  be  formed 
between  cc  and  a  medium  grating  will  produce  a 
spectrum  at  bb.  If  the  three  gratings  have  different 
rulings,  the  eye  at  E  will  see  the  lens  face  illuminated 
by  a  monochromatic  color  depending  upon  the  grating 
interposed  at  G.  If  all  three  spectra  be  produced 
simultaneously  in  proper  intensities,  the  eye  at  E 
would  see  the  lens  face  illuminated  by  white  light 
providing  gratings  of  proper  rulings  are  chosen.  Such 
a  scheme  was  used  by  Wood  for  viewing  the  photo- 
graphs. The  latter,  which  appear  colorless,  really 
consist  of  images  of  the  object  made  on  plates  of 
bichromated  gelatine  through  three  properly  chosen 


COLOR  PHOTOGRAPHY  217 

gratings.  In  making  the  photographs  one  of  the 
gratings  was  placed  in  contact  with  the  bichromated 
gelatine  film  and  the  image  of  the  object  was  pro- 
jected upon  the  sensitive  film  through  the  grating. 
This  grating  was  then  replaced  by  another  and  the 
procedure  repeated.  It  was  then  repeated  a  third 
time  with  the  remaining  grating,  but  usually  with 
another  sensitive  plate.  On  superposing  the  two 
exposed  plates  and  viewing  by  a  proper  combination 
of  lens  and  light  source  the  picture  was  seen  in 
colors.  Copies  can  be  made  by  contact  printing. 
It  was  found,  however,  that  while  satisfactory  pictures 
could  be  made  there  was  no  certainty  about  obtain- 
ing them.  This  was  later  found  to  be  due  to  super- 
posing the  three  grating  exposures.  Ives11  improved 
the  process  by  printing  the  grating  pictures  through 
a  very  coarse  grating  placed  at  right  angles  to  the 
lines  of  the  three  gratings.  The  coarse  grating  had 
opaque  lines  twice  the  width  of  the  transparent  strips. 
After  making  an  exposure  through  one  of  the  gratings 
the  coarse  auxiliary  grating  was  moved  in  a  direction 
perpendicular  to  its  lines  a  distance  equal  to  the 
width  of  one  of  its  open  slits  and  an  exposure  was 
made  through  the  second  grating.  This  procedure 
was  repeated  for  the  third  grating.  The  lines  of  the 
coarse  auxiliary  grating  were  as  in  the  so-called  Joly 
process  to  be  discussed  later,  so  narrow  as  to  be  just 
unresolved  by  the  eye  —  about  2*0  to  the  inch.  This 
process  yielded  satisfactory  results.  Ives  further 
simplified  the  process  by  making  one  grating  answer 
the  purpose  of  the  original  three  by  using  the  finest 
grating  —  3600  lines  per  inch  —  and  rotating  it  in 
its  plane  respectively  21.5  and  42  degrees  for  the 
other  two  exposures.  Thorp,  unknown  to  Ives,  had 
previously  suggested  the  use  of  one  grating  for  a 


218  COLOR  AND   ITS  APPLICATIONS 

similar  purpose.  When  using  three  gratings,  one 
with  2400  lines  per  inch  furnished  the  red  component, 
one  with  3000  the  green,  and  one  with  3600  the  blue. 
F.  E.  Ives  worked  out  a  viewing  apparatus  involving 
important  improvements  over  Wood's  original  scheme. 
60.  Color  Filter  Processes.  —  If  an  object  be 
separately  photographed  on  three  panchromatic  plates 
respectively  through  properly  chosen  red,  green,  and 
blue  filters  (which  collectively  transmit  all  visible 
rays)  and  these  three  photographs  be  separately 
projected  upon  a  white  screen  by  means  of  three 
projection  lanterns  equipped  with  the  foregoing  colored 
filters,  three  separate  '  monochromatic '  photographs  will 
be  seen.  If,  however,  the  three  colored  images 
-red,  green,  and  blue  —  be  superposed  in  exact 
coincidence,  a  picture  in  natural  colors  will  be  seen. 
The  principle  is  that  of  adding  colors  as  shown  in 
Fig.  21.  F.  E.  Ives,  who  was  a  pioneer  in  this  field, 
developed  an  apparatus  for  viewing  the  three-colored 
photographs  simultaneously  and  also  the  so-called 
chromoscope  for  tri-color  projection  of  photographs 
made  in  this  manner.  Charles  Cros  independently 
developed  a  similar  method.  In  1868  Louis  Ducos  du 
Hauron  described  a  process  for  three-color  photog- 
raphy (since  known  as  the  Joly  process)  which  in- 
volved the  ruling  of  red,  green,  and  blue  lines  of 
transparent  dyes  on  a  transparent  screen.  The  lines 
were  too  fine  to  be  distinguished  by  the  eye.  The 
procedure  involves  the  juxtapositional  method  of 
color-mixture,  a  principle  long  used  in  the  textile 
industry  and  in  painting.  If  a  photograph  be  made 
through  such  a  screen  and  a  positive  made  there- 
from, the  latter  will  appear  in  colors  when  viewed 
through  the  original  screen  when  properly  superposed. 
The  screen  is  diagrammatically  shown  largely  mag- 


COLOR  PHOTOGRAPHY 


219 


nified  in  Fig.  96.  (In  Figs.  96,  97,  and  98  red,  green, 
and  blue  are  represented  respectively  by  the  hori- 
zontal lines  and  the  diagonal  lines  running  in  direc- 
tions perpendicular  to  each  other.)  There  have  been 
numerous  variations  of  this  scheme  commercialized. 
The  Paget  screen  is  illustrated  in  Fig.  97.  An  inter- 
esting development  is  the  Lumiere  process.  Minute 
grains  of  dyed  starch  are  used  in  a  thin  layer  over 
a  sensitive  emulsion.  Three  batches  of  transparent 
starch  grains  are  «dyed  respectively  orange-red,  green, 


du  Hauron 


Paget 


Lumiere 


FIG.  96 


FIG.  97 


FIG.  95. 


Green          Red  Blue 

Illustrating  three  processes  of  color  photography. 

and  blue.  These  are  mixed  in  such  proportions  as 
to  give  a  mixture  of  neutral  color  and  are  spread  on 
the  plate  in  a  single  layer.  A  portion  of  the  plate 
greatly  magnified  is  shown  diagrammatically  in  Fig. 
98.  The  light  passes  through  the  minute  color  filters 
of  dyed  starch  before  striking  the  plate.  The  plate 
is  developed  in  the  ordinary  manner,  and  by  chemical 
means  the  negative  is  converted  into  a  positive.  The 
reversal  may  take  place  in  a  bath  of  potassium  per- 
manganate acidified  with  sulphuric  acid  and  is  later 
developed  again  in  the  same  developer  as  used  in 
the  first  development.  After  drying,  the  plate  is 
varnished  and  the  color  photograph  is  ready  for 


220  COLOR  AND  ITS  APPLICATIONS 

viewing.  The  process  is  a  very  ingenious  one  and 
reproduces  natural  colors  quite  satisfactorily.  A  de- 
ficiency of  the  process  of  no  great  importance  in  most 

work  is  shown  in  Fig.  99. 
It  is  also    of  interest   in 
showing   the   inability  of 
0.40  050  0.60  0.70    the  eye  to  analyze  colors. 

^.  WAVE  LENGTH  The    approximate    trans- 

Fig.  99. -nitrating  the  limitations  of    mission  of  the  three  dyes 

certain  processes  of  color  photography. 

are    diagrammatically 

shown.  It  is  evident  on  photographing  the  solar 
spectrum  that  the  dyes  used  are  somewhat  too 
monochromatic,  because  the  colored  spectrogram 
which  consists  of  red,  green,  and  blue  bands 
shows  gaps  between  the  blue  and  green,  and 
also  between  the  green  and  red  where  little  color 
is  visible.  For  instance  in  a  spectrogram  of  the 
mercury  spectrum  the  yellow  lines  at  0.578/x  appear 
an  orange-red.  This  defect  is  evident  in  a  greater 
or  less  degree  in  the  foregoing  processes,  depending 
upon  the  spectral  character  of  the  colored  dyes. 
However,  owing  to  the  fact  that  colors  ordinarily  en- 
countered are  far  from  monochromatic,  this  deficiency 
is  unimportant  and  practically  negligible  in  ordinary 
color  photography.  This  defect  is  encountered  with 
regret  when  one  desires  to  reproduce  spectra  for 
demonstration  purposes.  For  the  latter  purpose  the 
methods  employing  the  three  colored  transparencies 
about  to  be  described  are  satisfactory.  The  fore- 
going methods  are  based  upon  the  additive  and  juxta- 
positional  processes  of  color-mixture.  The  processes 
using  the  minute  color  filters  shown  in  Figs.  96,  97, 
and  98  have  a  disadvantage  in  loss  of  light.  For 
instance,  if  the  process  be  analyzed  it  will  be  seen 
that  a  red  object  will  be  recorded  upon  the  photo- 


COLOR  PHOTOGRAPHY  221 

graph  in  general  in  the  proportion  of  one  red  patch 
to  two  black  patches.  That  is,  no  red  light  will  be 
transmitted  by  the  minute  blue  and  green  filters,  so 
in  the  final  photograph  these  will  appear  as  black 
spots.  This  is  a  decided  disadvantage  in  the  making 
of  colored  lantern  slides  for  projection  unless  an 
exceedingly  powerful  arc  lamp  is  available.  A  num- 
ber of  processes  employing  subtractive  method  (#1.8) 
have  been  developed.  Sanger  Shepherd  developed 
a  method  wherein  three  differently  colored  films  such 
as  are  indicated  in  Fig.  20  are  superposed  in  a  single 
transparency.  F.  E.  Ives  was  also  a  pioneer  in  this 
field.  The  process  is  identical  in  principle  with  the 
tri-color  printing  process  in  use  at  the  present  time, 
with  the  exception  that  in  the  latter  case  a  black- 
white  record  is  sometimes  used  with  the  three  color 
records.  Three  negatives  are  made  respectively 
through  red,  green,  and  blue  filters  from  which  posi- 
tives are  made  on  special  thin  transparent  films  of 
celluloid  coated  with  gelatine  sensitized  by  immersion 
in  a  solution  of  bichromate  of  potash.  The  trans- 
parencies are  each  dyed  a  color  complementary  to 
that  of  the  taking  filter,  the  red  record  being  colored 
blue-green  (cyan-blue);  the  green  record,  purple 
(magenta);  and  the  blue  record,  yellow.  These 
transparencies  are  free  from  opaque  silver  deposit, 
the  gradation  being  from  a  maximal  transparency  to 
the  deepest  color  of  the  dye  on  each  film.  On  super- 
posing them  the  natural  colors  are  produced  by  the 
subtractive  method,  as  will  be  readily  understood 
from  an  inspection  of  Fig.  20.  F.  E.  Ives  devised  a 
method  after  this  principle  whereby  the  three  plates 
were  exposed  simultaneously  with  one  lens.  Shep- 
herd first  employed  a  repeating  plate  holder  so  that 
the  three  plates  were  successively  exposed  through 


222  COLOR  AND  ITS  APPLICATIONS 

the  proper  filters.  A  few  years  ago  a  process  of 
producing  moving  pictures  in  colors  known  as  Kinema- 
color  was  launched.  In  order  to  simplify  the  matter 
only  two  colors  are  used,  namely  a  blue-green  and  an 
orange-red.  The  different  colored  images  are  alter- 
nately thrown  on  the  screen  at  the  usual  rate.  It 
is  obvious  that  the  use  of  three  colors  would  render 
the  problem  exceedingly  complex.  Such  a  two- 
color  method  cannot  reproduce  all  colors  with  fidelity, 
but  the  results  are  quite  satisfactory  considering  the 
simplification  that  is  obtained.  Recently  another 
scheme,  employing  only  two  colors,  has  been  devel- 
oped, known  as  the  Kodachrome  process.  By  means 
of  a  repeating  back  two  plates  are  successively  ex- 
posed through  red  and  green  filters  respectively. 
These  are  developed  in  the  ordinary  manner  and 
after  being  washed  they  are  bleached  and  fixed,  at 
this  stage  appearing  transparent.  They  are  next 
given  a  final  washing  in  a  weak  aqueous  solution  of 
ammonia  and  dried.  Finally  the  plates  are  dyed, 
the  one  made  through  the  red  filter  being  dyed  a 
bluish-green  and  the  one  made  through  the  green 
filter  an  orange-red. 

In  general  the  processes  employing  dyed  trans- 
parencies superposed  yield  more  brilliant  color  records, 
but  are  obviously  more  dependent  upon  the  skill  of 
the  photographer.  In  much  work  the  processes 
employing  the  juxtapositional  method  of  color-mixture 
are  more  satisfactory  owing  to  the  simplicity,  notwith- 
standing the  less  brilliant  results.  Of  the  latter 
methods  those  employing  the  ruled  screens  are  some- 
what more  flexible;  however,  the  adjustment  of  the 
viewing  screens  requires  some  patience. 

It  is  thus  seen  that  at  the  present  time  the  prob- 
lem of  color  photography  has  been  solved  by  rather 


COLOR  PHOTOGRAPHY  223 

indirect  methods  involving  color-mixture.  Most  of 
the  methods  will  be  completely  understood  on  refer- 
ring to  Chapter  III. 

REFERENCES 

1.  Wiedmann's  Ann.  1895,  p.  335. 

2.  Amer.  Jour.  Sci.  1887,  p.  349. 

3.  Ann.  d.  Chimie  et  Phys.  1848,  p.  451. 

4.  Comp.  Rend.  114,  p.  961;  111,  p.  575. 

5.  Lehrbuch  der  Photochrome,  1868. 

6.  Ann.  d.  Phys.  1899,  69,  p.  488. 

7.  Des  Farbenphotographie  nach  Lippmann's  Verfahren,  1898. 

8.  Die  Photographic  in  naturlichen  Farben,  1894. 

9.  Beitrage  zur  Theorie  und  Praxis  der  director  Farben-photo- 
graphie,  1906. 

10.  Astrophys.  Jour.  1905,  27,  p.  325. 

11.  Jour.  Franklin  Inst.  June,  1906. 

OTHER    REFERENCES 

Louis  Ducos  du  Hauron,  Les  Colours  en  Photographie,  1868. 

R.  Child  Bailey,  Photography  in  Colours,  1900. 

E.  Konig,  Natural  Color  Photography,  1906. 

E.  Konig,  Beiblatter  Ann.  d.  Phys.  1909,  p.  1027. 

J.  A.  Starcke,  Sci.  Amer.  Sup.  Mar.  9,  1913,  p.  158. 

A.  Byk,  Phys.  Zeit.  Nov.  22,  1909,  p.  921. 

G.  E.  Brown,  Photo  Miniature,  No.  128,  1913. 

G.  L.  Johnson,  Photography  in  Colours,  1914. 


CHAPTER   XI 
COLOR  IN  LIGHTING 

61.  Lighting  is  of  great  importance,  because  it  is 
essential  to  our  most  important  and  educative  sense 
—  vision  —  and  color  is  intimately  associated  with 
lighting  and  vision.  Color  in  lighting  is  rapidly  grow- 
ing in  interest  in  the  science  and  art  of  illumination. 
The  recent  increase  in  the  luminous  efficiency  of 
light  sources  and  the  rapid  strides  in  the  development 
of  the  art  of  lighting  are  largely  responsible  for  the 
growing  interest  in  color  and  quality  of  light.  Much 
is  yet  to  be  learned  regarding  the  physiological  and 
psychological  effects  of  color,  and  the  laws  for  its 
proper  use  are  hazy  and  not  well  understood.  How- 
ever, equipped  with  a  full  knowledge  of  the  physics 
of  color,  an  aesthetic  taste  and  a  comprehensive  view 
of  what  is  known  and  unknown  regarding  the  physio- 
logical and  psychological  influence  of  color,  a  person 
is  capable  of  utilizing  many  of  the  possibilities  of 
color  in  lighting.  The  illuminant  plays  a  very  im- 
portant part  in  the  appearance  of  colors,  as  has  been 
seen  in  Chapter  VII.  The  spectral  character  of  the 
illuminant  influences  the  hue  and  relative  brightness 
of  colors,  and  the  intensity  influences  the  hue  and 
apparent  saturation.  At  low  intensities  the  hue 
shifts  toward  the  shorter  wave-lengths  and  at  high 
intensities  there  is  an  apparent  decrease  of  satura- 
tion. The  distribution  of  the  light  affects  the  appear- 
ance of  colors,  owing  to  the  character  of  these 
surfaces.  All  of  these  factors  are  of  importance  in 

224 


COLOR  IN  LIGHTING  225 

considering  the  proper  illuminant  for  accurate  color 
work  in  the  dye-rooms  of  textile  and  paper  mills,  in 
the  mixing  of  pigments  for  color  printing  and  for 
painting,  for  the  matching  of  colors,  and  in  many 
other  places.  The  spectral  character  of  illuminants 
is  of  importance  (#37)  in  the  discrimination  of  fine 
detail,  for  it  has  been  seen  that  monochromatic  light 
is  superior  in  defining  power  to  light  of  any  other 
spectral  character. 

There  are  many  important  problems  as  yet  un- 
solved which  involve  color  in  its  application  to  light- 
ing. There  are  practically  no  data  on  the  influence 
of  color  on  eye  fatigue,  although  it  is  known  that 
colors  are  of  influence  psychologically.  There  is  a 
prevalent  idea  that  the  kerosene  lamp  is  'easy  on  the 
eyes,'  owing  to  its  yellowish  color.  However,  the 
low  intrinsic  brightness  of  the  kerosene  flame  as 
compared  with  more  modern  illuminants  is  a  fact 
worthy  of  consideration.  When  it  is  further  noted 
that  there  is  no  general  objection  to  daylight  on 
account  of  its  color  —  and  it  is  far  whiter  or  more 
bluish  than  ordinary  illuminants  —  it  must  be  ad- 
mitted that  the  virtue  of  the  kerosene  lamp  based 
upon  its  color  is  on  a  rather  shaky  foundation.  It 
is  likely  that  the  eye  having  evolved  under  daylight 
is  better  adapted  to  it  than  to  any  other  illuminant 
and  that  the  nearer  an  artificial  illuminant  approaches 
daylight  in  spectral  character  the  more  likely  is  it  to 
be  satisfactory  physiologically.  Misuse  of  common 
illuminants  is  perhaps  responsible  for  eye-fatigue  to 
a  greater  extent  than  any  spectral  characteristics. 
One  cannot  look  directly  at  the  sun  and  state  con- 
scientiously that  daylight  is  ideal.  It  has  been  found 
that  visual  acuity  is  better  in  monochromatic  light 
than  in  daylight  (#37),  and  it  may  appear  from  this 


226  COLOR  AND  ITS  APPLICATIONS 

that  daylight  is  not  ideal.  However,  these  experi- 
ments were  carried  out  at  ordinary  intensities  con- 
sidered satisfactory  in  artificial  lighting,  and  daylight 
intensities  are  ordinarily  very  much  greater,  which 
means  that,  for  the  discrimination  of  ordinary  details, 
the  intensity  is  many  times  the  minimal  amount  re- 
quired, so  that  the  limit  of  defining  power  is  seldom 
reached.  For  years  many  have  held  that  the  eyes 
are  less  fatigued  when  reading  from  yellow  paper 
than  from  white  paper.  In  a  biography  of  Joaquin 
Miller  we  read  that  'he  wrote  on  yellow  paper  with 
a  pencil  because  white  paper  hurt  his  eyes.'  Bab- 
bage  many  years  ago  strongly  advocated  the  use  of 
yellowish  paper  in  reference  books,  such  as  logarithm 
tables,  where  the  eyes  are  severely  taxed.  Javel 
later  advocated  the  same  procedure,  claiming  that 
eye-strain  was  decreased,  owing  to  a  decrease  in 
contrast.  Many  are  of  the  same  opinion  although, 
as  already  stated,  quantitative  data  relating  directly 
to  the  problem  are  lacking.  After  reading  from  white 
paper  the  eyes  seem  to  welcome  a  change  to  yellow 
paper,  but  this  may  be  due  to  a  decrease  in  contrast, 
owing  to  a  lower  reflection  coefficient  of  the  yellow 
paper  than  that  of  the  white  paper.  However,  meas- 
urements show  only  a  slight  difference  in  the  bright- 
ness of  pale  yellow  copy  paper  as  compared  with  white, 
especially  under  ordinary  artificial  light.  There  is 
no  doubt  that  a  yellow  or  yellow-green  light  of  less 
extended  spectral  character  than  daylight  or  ordinary 
artificial  light  is  of  superior  defining  power,  due  to 
the  reduction  of  the  effects  of  chromatic  aberration 
in  the  eye.  This  fact  may  partly  account  for  the 
contention  that  yellow  paper  is  *  easier  on  the  eyes.' 
It  is  difficult  to  focus  blue  light  at  a  normal  reading 
distance,  and  impossible  to  do  this  at  the  same  time 


COLOR  IN  LIGHTING  227 

keeping  the  most  luminous  rays  in  focus,  therefore, 
the  elimination  of  the  blue  rays  by  means  of  yellow 
paper  may  actually  increase  the  definition.  However, 
reading  does  not  ordinarily  involve  the  discrimination 
of  fine  detail,  but  instead  the  recognition  of  groups 
of  characters.  Furthermore,  the  eye  is  found  to  pro- 
gress across  a  page  in  a  series  of  jumps,  being  sta- 
tionary only  a  few  times  per  line.  It  has  been  found 
that  there  is  practically  no  difference  in  visual  acuity 
when  the  detail  is  viewed  against  a  white  ground 
and  a  ground  consisting  of  yellow  copy  paper  when 
both  receive  the  same  intensity  of  illumination,  that 
is  the  same  density  of  light  flux. 

Colored  surroundings,  such  as  foliage,  brick  walls, 
the  wall  coverings  of  the  room,  etc.,  alter  the  spectral 
character  of  light  before  it  arrives  at  the  useful  plane. 
Such  effects  must  be  considered  in  any  lighting  prob- 
lem requiring  a  light  of  a  certain  spectral  quality  and 
are  also  of  importance  from  the  aesthetic  viewpoint. 
Many  uses  of  illuminants  of  different  color  and  colored 
media  are  found  in  the  problems  of  lighting. 

62.  The  Production  of  Artificial  Daylight. —The 
arts  having  developed  largely  under  daylight  illumi- 
nation, the  daylight  appearance  of  colors  is  naturally 
considered  as  standard.  With  the  production  of 
artificial  light  man  became  less  dependent  upon  day- 
light; nevertheless,  owing  to  the  impracticability  and 
perhaps  impossibility  of  a  dual  criterion  of  color,  there 
has  always  been  a  demand  for  artificial  daylight. 
The  efforts  in  the  production  of  artificial  light  have 
been  directed  toward  the  production  of  light  of  day- 
light spectral  quality.  The  principal  reason,  no  doubt, 
is  that  such  a  procedure  in  our  most  important  method 
of  producing  light  (by  high  temperature  radiation) 
at  the  present  time  tends  toward  an  ever-increasing 


228  COLOR  AND  ITS  APPLICATIONS 

luminous  efficiency.  Nevertheless  each  increment  in 
the  steady  approach  toward  daylight  has  been  loudly 
acclaimed  by  reason  of  the  better  '  color-value '  of  the 
illuminant.  However,  there  is  a  method  which  has 
been  applied  whereby  light  of  a  daylight  character 
can  be  obtained  by  excluding  from  an  illuminant  con- 
taining all  the  rays  found  in  daylight,  those  portions 
which  are  present  in  excessive  amounts.  Such  a 
subtractive  method  is  wasteful  of  light,  but  is  made 
practicable  by  the  recent  increase  in  the  luminous 
efficiency  of  illuminants.  However,  it  is  well  to 
remember  that  efficiency  in  lighting  as  in  any  other 
case  is  'the  ratio  of  satisfactoriness  to  cost  and  not 
the  reciprocal  of  the  cost.' 

In  order  to  produce  artificial  daylight  it  is  neces- 
sary to  determine  the  spectral  character  of  natural 
daylight.  First  it  is  well  to  distinguish  between  sun- 
light and  skylight.  The  latter  is  scattered  sunlight, 
but  owing  to  the  relatively  greater  scattering  of  the 
rays  of  short  wave-length  (#13)  skylight  is  more 
bluish  in  color  than  sunlight.  Daylight  varies  tre- 
mendously with  time  and  place,  although  north  blue 
skylight  and  clear  noon  sunlight,  when  unaltered  by 
reflection  from  immediate  surroundings,  are  fairly 
constant  in  color.  However,  the  modification  due 
to  selective  absorption  of  the  particles  in  the  atmos- 
phere and  selective  reflection  from  foliage,  buildings, 
etc.,  make  daylight  rather  indefinite  in  spectral  char- 
acter. E.  L.  Nichols  1  has  published  interesting 
accounts  of  his  investigations  on  the  spectral  char- 
acter of  daylight  under  different  conditions  of  weather, 
cloudiness,  location,  and  time  of  day.  He  found 
among  other  things  unmistakable  evidence  of  the 
coloring  added  to  daylight  by  reflection  from  green 
foliage  by  noting  the  characteristic  absorption  spec- 


COLOR  IN  LIGHTING  229 

trum  of  chlorophyl  (a  substance  in  green  foliage) 
present  in  observations  made  on  land  in  the  summer 
time.  This  effect  was  absent  on  the  sea.  Koettgen,2 
Nichols  and  Franklin,3  Crova,4  Vogel,5  Ives,6  and 
others  have  studied  the  spectral  character  of  day- 
light. The  data  on  noon  sunlight  and  skylight  plotted 
in  Fig.  5  is  a  weighed  mean  of  the  results  of  the 
foregoing  investigators  as  presented  by  Ives.  The 
distribution  of  energy  in  the  visible  spectrum  of 
clear  noon  sunlight  as  it  reaches  the  earth  corre- 
sponds closely  to  that  of  a  black  body  at  5000  deg. 
absolute  (C). 

A  number  of  investigators,  including  Dufton  and 
Gardner,7  Mees,8  Pirani,  Ives,9  Hussey,10  and  Luckiesh11 
have  devised  colored  screens  for  producing  artificial 
daylight  by  altering  the  light  from  an  artificial  source 
emitting  a  continuous  spectrum.  In  order  to  demon- 
strate the  procedure  and  illustrate  the  advantage  of 
first  choosing  a  light  as  close  to  daylight  as  possible, 
the  production  of  daylight  screens  for  two  tungsten 
lamps  of  different  luminous  efficiencies  as  considered 
by  Luckiesh  and  Cady  n  will  be  presented. 

The  visible  spectrum  of  the  light  from  a  tungsten 
lamp  being  continuous,  it  has  all  the  rays  present  that 
are  found  in  daylight.  The  difference  in  their  spec- 
tral characters  is  due  to  the  difference  in  the  relative 
amounts  of  the  various  rays  present.  First  let  us 
consider  the  production  of  light  of  noon  sunlight 
quality  from  a  vacuum  tungsten  incandescent  lamp 
operating  at  7.9  lumens  per  watt  (1.25  w.p.m.h.c.). 
It  is  found  sufficiently  accurate  to  consider  no  rays  of 
shorter  wave-length  than  0.42/z.  An  ideal  screen  for 
altering  the  tungsten  light  to  a  noon  sunlight  quality 
will  therefore  transmit  all  the  rays  of  wave-length 
0.42^.  It  will  partially  absorb  rays  of  longer  wave- 


230 


COLOR  AND   ITS  APPLICATIONS 


length  in  increasing  proportions  from  0.42^  toward 
the  long-wave  end  of  the  spectrum.  The  reduction 
of  the  intensity  of  the  rays  of  various  wave-lengths 
is  readily  computed  from  the  ratios  of  the  amounts 
of  these  rays  present  in  noon  sunlight  to  the  amounts 
of  the  corresponding  rays  present  in  the  tungsten 
light  under  consideration.  The  resultant  transmis- 
sion curve  of  a  colored  screen  for  thus  altering  the 


Fig.  100. — Ideal  transmission  screens  for  producing  artificial  daylight. 

tungsten  light  (7.9  lumens  per  watt)  to  noon  sunlight 
quality  is  shown  in  b,  Fig.  100.  The  ideal  transmis- 
sion curve  of  a  colored  screen  for  producing  artificial 
noon  sunlight  by  means  of  a  nitrogen-filled  tungsten 
lamp  operating  at  22  lumens  per  watt  (0.5  w.p.m.h.c.) 
is  shown  in  c.  In  order  to  produce  artificial  north 
skylight  it  is  seen  in  Fig.  5  that  the  visible  rays  of 
long  wave-length  must  be  reduced  by  relatively 
greater  amounts  than  in  producing  artificial  noon 
sunlight.  The  ideal  transmission  curve  for  producing 
artificial  north  skylight  by  means  of  the  tungsten 


COLOR  IN  LIGHTING 


231 


lamp  operating  at  7.9  lumens  per  watt  is  shown  in  a. 
The  ideal  transmission  curve  for  producing  artificial 
north  skylight  with  the  gas-filled  tungsten  lamp  oper- 
ating at  22  lumens  per  watt  coincides  closely  with  b. 
That  is,  a  screen  which  produces  artificial  noon  sun- 
light with  the  older  type  of  tungsten  lamp  operating 
at  7.9  lumens  per  watt  will  produce  artificial  skylight 
when  used  with  the  gas-filled  tungsten  lamp  operat- 


100 


0.40        044 


0.65 


0.72 


Fig.  101.  —  Showing  the  loss  of  light  when  using  the  ideal  artificial-daylight 
screens  with  the  tungsten  lamp  operating  at  7.9  lumens  per  watt. 

ing  at  22  lumens  per  watt.  This  fact  has  been  taken 
advantage  of  by  the  author  in  developing  daylight 
units.  These  curves  show  the  increased  daylight 
efficiency  of  the  tungsten  lamps  operating  at  higher 
luminous  efficiencies.  This  is  further  illustrated  in 
Figs.  101  and  102.  In  the  former  El  represents  the 
luminosity  curve  of  the  eye  for  light  from  a  tungsten 
lamp  operating  at  7.9  lumens  per  watt,  that  is,  the 
relative  light  values  of  the  rays  of  various  wave- 
lengths. This  curve  may  be  found  directly  or  by  mul- 


232 


COLOR  AND   ITS  APPLICATIONS 


tiplying  the  mean  luminosity  curve  of  the  eye  (Fig.  93) 
for  equal  amounts  of  energy  of  all  wave-lengths  by 
the  amounts  of  energy  of  various  wave-lengths  in 
the  spectrum  of  the  light  under  consideration.  In 
this  case  it  is  the  7.9  lumens  per  watt  tungsten 
lamp  whose  spectral  energy  distribution  is  found  in 
Fig.  5.  On  multiplying  curve  £1  by  the  transmission 
values  of  curve  a,  Fig.  100,  curve  a'  is  obtained.  The 


0.65        072. 


Fig.  102.  —  Showing  the  loss  of  light  when  using  the  ideal  artificial-daylight 
screens  with  the  tungsten  lamp  operating  at  22  lumens  per  watt. 

areas  under  curve  Ei  and  a'  are  proportional  to  total 
luminous  sensations,  and  the  ratio  of  the  area  of  a' 
to  that  of  Ei  represents  the  skylight  efficiency  of  the 
7.9  lumens  per  watt  tungsten  lamp  as  based  upon 
the  foregoing  computations.  The  reduction  in  lumi- 
nous intensity  when  screen  b  is  used  with  the  source 
under  consideration  is  found  on  comparing  b'  with 
E^  in  Fig.  101,  and  the  ratio  of  the  areas  represents 
the  sunlight  efficiency  of  the  7.9  lumens  per  watt 
tungsten  lamp.  The  corresponding  data  for  screens 


COLOR  IN  LIGHTING  233 

b  and  c  used  with  the  22  lumens  per  watt  gas-filled 
tungsten  lamp  are  shown  in  Fig.  102,  where  E2  repre- 
sents the  luminosity  curve  of  the  eye  for  this  tung- 
sten light.  Screen  b  produces  skylight  and  reduces 
the  luminous  intensity  an  amount  represented  by  the 
difference  between  the  area  of  b'  and  E2  in  Fig.  102. 
Screen  c  produces  noon  sunlight  with  an  efficiency 
represented  by  the  ratio  of  the  area  of  c'  to  that  of  E«. 

The  daylight  efficiencies  for  the  two  lamps  con- 
sidered in  the  foregoing  were  found  by  determining 
the  relative  areas.  For  the  7.9  lumens  per  watt 
tungsten  lamp  (vacuum  type)  the  noon  sunlight 
efficiency  is  14%  and  the  skylight  efficiency  4%. 
However,  for  the  22  lumens  per  watt  tungsten  lamp 
(nitrogen-filled  type)  the  corresponding  values  are 
considerably  higher,  being  25%  and  13%  respectively. 
It  has  been  found  in  actual  practise  that  the 
consideration  of  0.42/z  as  the  starting  point  for  the 
computations  just  described  conduces  to  a  higher 
accuracy  than  necessary  in  most  cases,  therefore 
beginning  with  a  screen  of  100%  transmission  at 
0.45/x  the  daylight  efficiencies  are  very  considerably 
increased.  Under  these  circumstances  for  the  7.9 
lumens  per  watt  lamp  the  sunlight  and  skylight 
efficiencies  are  respectively  18%  and  9%  and  for  the 
22  lumens  per  watt  lamp  33%  and  19%. 

It  is  thus  seen  that  very  accurate  artificial  noon 
sunlight  can  be  obtained  with  an  ideal  colored  trans- 
mission screen  with  the  22  lumens  per  watt  lamp  at 
an  efficiency  of  25%  or  at  5.5  lumens  per  watt.  This 
is  a  higher  efficiency  than  that  of  the  ordinary  carbon 
incandescent  lamp  operating  normally  at  the  present 
time.  Artificial  daylight  sufficiently  accurate  for 
nearly  all  purposes  can  be  made  at  a  much  higher 
efficiency.  The  author  has  developed  bulbs  for  the 


234  COLOR  AND   ITS  APPLICATIONS 

high  efficiency  tungsten  lamp  that  produce  artificial 
daylight  satisfactory  for  general  illuminating  pur- 
poses. Thus  the  advent  of  the  high  efficiency  lamps 
has  made  artificial  daylight  available,  and  now  that  it 
is  practicable  it  is  surprising  how  many  places  are 
found  for  it.  Besides  in  the  general  field  of  store 
lighting,  artificial  daylight  is  useful  for  mixing  pig- 
ments, matching  artificial  teeth  and  buttons,  cigar 
sorting,  medical  examination  of  manifestations  of 
skin  diseases,  green  houses  where  botany  classes 
study  at  night,  observations  of  chemical  reactions, 
and  for  many  other  operations. 

The  production  of  colored  media  for  the  above 
purpose  requires  spectrophotometric  apparatus.  Mis- 
takes have  been  made  by  using  colorimeters  or  by 
using  merely  the  eye  to  judge  the  color.  As  has 
already  been  seen,  the  eye  is  undependable  for  such 
purposes,  because  it  is  not  an  analytical  instrument 
for  the  examination  of  color.  Two  lights  may  appear 
white  to  the  eye,  yet  differ  considerably  in  spectral 
character.  For  instance,  ultramarine  blue  of  a  proper 
density  will  so  alter  tungsten  light  by  transmission 
that  a  white  paper  will  appear  quite  the  same  as 
under  daylight,  yet  colored  objects  will  appear  greatly 
different.  Such  a  screen  is  very  useful  for  demon- 
stration purposes.  The  distribution  of  energy  in 
the  visible  spectrum  of  a  white  light  produced  with 
an  ultramarine  filter  screening  a  tungsten  lamp 
operating  at  10  lumens  per  watt  as  compared  with  that 
of  noon  sunlight,  S,  is  shown  in  U,  Fig.  103.  This 
unit  was  once  seriously  proposed  as  a  'daylight  lamp,' 
but  was  short-lived  for  the  reason  shown.  Another 
white  light  is  shown  in  curve  C,  which  is  produced 
by  the  addition  of  red  and  blue-green  light.  It  is 
similar  to  the  ultramarine  white  light,  yet  more  ex- 


COLOR  IN  LIGHTING 


235 


treme.  These  three  illuminants  are  called  'white,' 
because  a  white  object  appears  the  same  under  all 
of  them;  however,  a  colored  object  does  not.  A 
quartz  mercury  arc  will  cause  a  white  paper  to  appear 
nearly  white,  yet  its  spectral  composition  is  known  to 
consist  chiefly  of  four  lines  in  the  visible  region. 


0.72 


Fig.  103.  —  Showing  the  spectral  analyses  of  two  subjective  white  lights  compared 
with  the  spectral  analysis  of  noon  sunlight. 

These  examples  illustrate  the  importance  of  spectro- 
photometric  measurements  in  such  problems. 

Another  method  of  producing  daylight  is  to  add  to 
a  continuous-spectrum  illuminant  the  correct  amounts 
of  certain  rays  which  are  not  present  in  sufficient 
amounts.  To  most  artificial  illuminants  of  this  char- 
acter violet,  blue,  and  blue-green  rays  must  be  added. 
To  illustrate  the  procedure  the  two  tungsten  lamps 
considered  previously  will  be  used.  In  Fig.  104  curve 
S  represents  the  spectral  distribution  of  energy  in 


236 


COLOR  AND   ITS  APPLICATIONS 


noon  sunlight.  Curves  A  and  B  represent  respec- 
tively the  spectral  distributions  of  energy  for  the  two 
tungsten  lamps  operating  at  7.9  and  22  lumens  per 
watt.  These  three  curves  are  plotted  with  their 
energy  values  equal  at  0.70/x,  a  point  near  the  prac- 
tical limit  of  visibility  for  long-wave  energy.  By 
subtracting  the  ordinates  of  A  and  B  respectively 


260 


Fig.  104.  —  Showing  the  additive  method  of  producing  artificial  daylight. 

from  the  ordinates  of  S  and  plotting  the  remainders, 
curves  A'  and  B'  are  obtained.  These  curves  are 
complementary  to  A  and  B  respectively;  that  is,  the 
light  produced  by  A  when  added  to  the  light  pro- 
duced by  A'  gives  the  same  amount  of  light  and  of 
exactly  the  same  spectral  character  as  the  light  pro- 
duced by  S,  which  is  assumed  to  be  white  light.  By 
multiplying  the  ordinates  of  S,  A,  and  B  by  the  light 
values  of  energy  of  corresponding  wave-lengths  the 
curves  in  Pig.  105  are  obtained.  For  example,  S  is 


COLOR  IN  LIGHTING 


237 


the  luminosity  curve  of  the  eye  for  noon  sunlight.  On 
integrating  these  curves  the  relative  areas  under  S, 
By  and  A  are  respectively  100,  50,  33.  Thus  it  is  seen 
that  equal  amounts  of  light  from  a  nitrogen-filled 
tungsten  lamp  operating  at  22  lumens  per  watt  and 
light  of  such  a  spectral  character  as  5',  Fig.  104,  will 
produce  artificial  noon  sunlight.  However  one  part 


240 
220 
200 
180 

UJ 

3160 

^140 
12120 
£100 
5  80 

LJ 

tt  60 
40 


20 


0.40 


\ 


044 


046 


\\ 


0.52 


0.56 


0.60         0.64 


07? 


Fig.  105.  —  Showing  the  relative  amounts  of  light  of  the  character  of  A  and  B 
(Fig.  104)  necessary  to  produce  artificial  daylight  by  addition. 

of  light  from  a  vacuum  tungsten  lamp  operating  at 
7.9  lumens  per  watt  must  be  added  to  two  parts  of 
light  of  the  character  of  A'y  Fig.  104,  to  produce 
artificial  noon  sunlight.  These  data  have  proved  of 
value  in  the  use  of  colored  lamps  with  clear  lamps 
for  the  lighting  of  paintings  and  other  decorative 
colored  objects. 

In  Table  VII  the  'per  cent  white'  values  obtained 
by  L.  A.  Jones  12  for  various  artificial  illuminants  with 
a  monochromatic  colorimeter  are  presented.  His 


238  COLOR  AND  ITS  APPLICATIONS 

values  show  higher  daylight  efficiencies  for  the  tung- 
sten incandescent  lamps  than  obtained  by  Luckiesh 
and  Cady.11  The  difference  may  be  partly  due  to  a 
difference  in  the  standards  of  white  light  used  and  in 
part  to  the  possible  fact  that  the  author's  computa- 
tions were  made  for  artificial  daylight  of  too  great 
accuracy.  That  is,  it  is  possible  that  the  extremely 
low  luminosity  of  rays  at  0.42/z  makes  it  unnecessary 
to  produce  a  screen  that  begins  to  absorb  light  at 
that  extremely  short  visible  wave-length.  The  com- 
putations for  screens  beginning  to  absorb  rays  of 
longer  wave-length  than  0.45/z  more  nearly  agree  with 
the  data  obtained  by  Jones.  It  is  unfortunate  that 
Jones  did  not  rate  his  tungsten  lamps  in  lumens  per 
watt,  which  is  more  definite  because  the  mean  hori- 
zontal candle-power  of  a  tungsten  lamp  depends 
so  much  upon  the  manner  of  mounting  the  filament. 
Ives 13  obtained  data  on  the  daylight  efficiency  of 
illuminants  several  years  ago,  but  his  standard  of 
daylight  used  at  that  time  does  not  agree  with  a 
standard  later  arrived  at  by  him  by  weighing  the 
observations  of  various  investigators,  so  that  m's 
values  are  not  presented  here. 

63.  Practical  Units  for  Imitating  Daylight.  —  Lu- 
minous efficiency  in  artificial  daylight  production  is 
a  minor  matter  in  a  unit  developed  for  very  accurate 
color-matching.  However,  there  are  many  cases  where 
light  approximating  daylight  quality  is  desired  for 
general  lighting.  Here  the  wattage  is  an  important 
consideration,  although  illuminating  engineers  and 
consumers  alike  must  learn  that  the  efficiency  of  a 
lighting  unit  or  installation  is  a  measure  of  how  well 
it  fulfills  its  purpose.  This  means  a  broader  concept 
than  watts  per  square  foot  or  effective  lumens  per 
watt.  If  a  light  source  is  used  for  illuminating  dress 


COLOR  IN  LIGHTING  239 

goods,  and  blues  cannot  be  distinguished  from  blacks, 
and  greens  as  seen  in  daylight  are  confused  with 
yellow  and  brown  fabrics  under  the  artificial  light, 
then  the  efficiency  of  the  lighting  installation  falls 
close  to  zero  in  these  particular  cases.  As  illuminat- 
ing procedure  becomes  more  refined,  and  as  the  effi- 
ciency of  light  production  increases,  more  attention 
is  being  given  to  the  importance  of  quality  of  light, 
which  is  an  important  factor  in  many  lighting  prob- 
lems. For  these  reasons  glassware  for  use  with 
tungsten  lamps  of  high  efficiency  was  developed  by 
the  author11  in  1914  which  greatly  improves  the  qual- 
ity of  the  light,  and  does  so  without  such  an  excessive 
loss  of  light  as  would  be  impractical  for  purposes  of 
general  lighting. 

Three  phases  of  daylight  have  been  considered, 
with  the  result  that  three  classes  of  units  have  been 
developed.  The  latest  color-matching  unit,  in  which 
the  gas-filled  tungsten  lamp  operating  at  22  lumens 
per  watt  is  used,  produces  light  of  a  deep  blue 
skylight  quality  at  about  3  lumens  per  watt.  With 
the  multiple  lamps  of  the  same  type  operating  at  15 
lumens  per  watt  the  light  corresponds  to  that  of  sky- 
light not  quite  as  blue,  and  the  luminous  efficiency  is 
about  2  lumens  per  watt.  This  unit  is  used  for  the 
purpose  of  accurate  discrimination  of  color  in  textile 
mills,  laboratories,  color-printing  shops,  etc.  The 
colored  screen  is  entirely  of  glass,  and  as  there  is  no 
excessive  temperature  rise  in  a  well-ventilated  unit, 
the  glass  is  permanent  and  the  unit  is  entirely  safe. 

The  next  class  of  units  are  intended  to  imitate 
clear  noon  sunlight.  This  might  be  considered  an 
average  outdoor  daylight.  There  are  many  cases 
indoors  where  the  daylight  quality  is  a  mixture  of 
sunlight  and  skylight,  and  this  unit  is  designed  to 


240  COLOR  AND   ITS  APPLICATIONS 

produce  a  satisfactory  artificial  sunlight  at  an  effi- 
ciency of  about  7  lumens  per  watt  when  multiple 
tungsten  lamps  operating  at  an  efficiency  of  about 
16.5  lumens  per  watt  are  used.  It  will  be  noted 
that  the  luminous  efficiency  at  which  this  artificial 
sunlight  is  produced  is  practically  the  same  as  that 
of  the  older  type  of  tungsten  lamps.  Thus  sunlight 
quality  is  available  for  general  lighting  purposes. 
The  applications  for  such  units  are  to  be  found  in 
color  factories,  lithographing  plants,  wall  paper  and 
paint  stores,  paint  shops,  cigar  factories,  art  galleries, 
etc. 

Other  units  have  been  made  by  combining  this 
colored  element  with  ornamental  glassware,  by  casing 
with  light-density  opal,  or  by  mixing  the  two  inti- 
mately. These  units  are  intended  for  use  in  general 
store  lighting,  where  a  better  quality  of  light  is  often 
desirable  than  can  be  obtained  from  any  practical 
light  source  available  for  general  store  lighting.  Any 
desired  step  toward  sunlight  quality  can  be  produced, 
the  magnitude  of  the  step,  of  course,  depending  upon 
the  permissible  overall  luminous  efficiency  and  the 
color  desired.  Notwithstanding  the  blue  or  white 
appearance  of  daylight,  when  such  a  quality  of  light 
is  produced  artificially,  there  is  some  objection  to 
its  use  in  stores  because  of  the  'cold'  appearance, 
notwithstanding  its  necessity  for  the  proper  appear- 
ance of  colors.  By  this  means  a  quality  of  light 
better  than  can  be  obtained  from  any  unaltered  light 
source  for  general  use  is  produced  at  a  luminous 
efficiency  sufficiently  high  to  meet  with  favor.  Ob- 
viously a  quality  of  light  approximately  midway  be- 
tween that  from  the  new  high  efficiency  tungsten 
lamps  and  sunlight  can  be  obtained  at  a  higher 
efficiency  than  that  of  the  older  types  of  tungsten 


COLOR  IN  LIGHTING  241 

lamps.  Lamps  operating  at  a  higher  efficiency  emit 
a  whiter  light,  to  begin  with,  thus  giving  the  gas- 
filled  tungsten  lamps  a  dual  advantage  over  those  of 
the  older  type  for  the  purpose  of  artificial  daylight 
production.  Recently  this  glass,  with  a  slight  modi- 
fication, has  been  incorporated  into  bulbs  for  the 
gas-filled  tungsten  lamps  for  the  purpose  of  general 
lighting. 

As  already  stated,  any  light  source  having  a 
continuous  spectrum,  or  one  nearly  so,  can  be  used 
for  the  purpose  of  making  artificial  daylight.  Other 
desirable  characteristics  are  high  luminous  efficiency 
and  steadiness  of  light  both  as  to  quality  and  in- 
tensity. The  arc  lamp  early  entered  the  field  and 
has  been  used  considerably,  although  fluctuations  in 
both  the  color  and  intensity  have  been  serious  draw- 
backs. A  unit  developed  by  Dufton  and  Gardner7 
in  1900  appears  to  be  the  first  practical  use  made  of 
the  colored  screen  for  subtractively  imitating  day- 
light. Doubtless  there  have  been  many  more  or  less 
approximate  reproductions  made  by  others. 

Many  are  familiar  with  the  beautiful  white  light 
of  the  Moore  carbon-dioxide  vacuum-tube  lamp.14 
No  better  approximation  of  average  daylight  could 
be  desired;  however,  at  present  the  luminous  effi- 
ciency of  the  small  units  for  color-matching  purposes 
is  quite  low.  Certain  difficulties  have  prevented  the 
general  adoption  of  the  longer  tube,  although  wher- 
ever this  unit  has  been  used  the  quality  of  the  light 
appears  to  be  very  satisfactory. 

In  1909  the  mercury  arc  lamp  was  combined  with 
the  tungsten  lamp  in  proper  proportions,  with  the 
result  that  a  white  light  was  produced.  However, 
this  is  only  an  approximate  imitation  of  daylight,  the 
blue  lines  of  the  mercury  spectrum  supplying  the 


242  COLOR  AND  ITS  APPLICATIONS 

blue  rays  in  which  the  old  tungsten  lamp  was  quite 
deficient.  This  combination  cannot  result  in  a  true 
daylight  as  considered  spectrally,  because  the  spectrum 
of  the  mercury  arc  consists  of  only  a  few  lines.  The 
addition  of  the  fluorescent  reflector  to  the  mercury 
vapor  lamp  greatly  improved  this  illuminant  by  adding 
red  rays,  but  this  is  done  partially  at  the  expense  of 
green  light.  (See  Figs.  4,  15,  and  16.) 

Early  in  1911  Ives  and  Luckiesh,9  by  means  of  two 
commercial  glasses  and  an  aniline  dye,  produced  a 
screen  for  use  with  the  old  tungsten  lamp  operating 
at  1.25  w.p.m.h.c.  for  the  purpose  of  producing  'aver- 
age daylight,'  that  is,  noon  sunlight.  Later  the  two 
glasses  were  replaced  by  a  single  glass,  but  a  cor- 
recting aniline  dye  was  still  necessary. 

In  1912  R.  B.  Hussey 10  described  a  screen  for 
use  with  an  intensified  arc  which  produced  sunlight 
quality.  This  was  done  by  means  of  pieces  from  two 
colored  glasses  arranged  in  a  checkerboard  fashion, 
with  suitable  diffusing  glasses  to  mix  the  light.  Owing 
to  the  unsteadiness  of  the  arc,  spectrophotometric 
measurements  were  difficult  to  make,  therefore  a 
colorimeter  developed  by  F.  E.  Ives  was  used  (#28, 
Fig.  53).  It  will  be  noted  that  colorimeter  measure- 
ments are  not  sufficiently  analytical  for  the  purpose 
of  determining  the  character  of  the  spectrum  of  a 
light  source.  For  instance,  this  instrument  will  indi- 
cate that  the  quartz  mercury  arc  gives  approximately 
white  light,  yet  this  light  source  emits  a  line  spectrum 
consisting  chiefly,  in  the  visible  region,  of  four  spec- 
tral lines,  as  shown  in  Fig.  4.  However,  the  colorim- 
eter measurements  are  of  interest  where  the  light 
is  known  to  have  an  approximately  continuous  spec- 
trum. This  instrument  gives  readings  in  terms  of 
red,  green,  and  blue  components,  which  when  mixed 


COLOR  IN  LIGHTING 


243 


produce   the   same   color   on   a  white   surface   as   the 
illuminant    under    examination.     In    Table    XVII    are 

TABLE   XVII 

Colorimeter  Measurements  on  Units  for  improving  the  Spectral 
Quality  of  Artificial  Light  toward  Daylight 


Source 


Colorimeter  reading 


Red      Green      Blue 


Average  daylight  (noonday  sunlight) 

North  blue  skylight 

Hussey  daylight  arc  lamp 

Intensified  arc  lamp  (bare) 

Ives  and  Luckiesh  (artificial  daylight) 

Tungsten  1.25  w.  p.  m.  h.  c.  (7.9  lumens  per  watt) 
Tungsten  0.65  w.  p.  m.  h.  c.  (16.4  lumens  per  watt)  .  . . 

Tungsten  0.50  w.  p.  m.  h.  c.  (22  lumens  per  watt) 

Tungsten  1.25  w.  p.  m.  h.  c.  in  tinted  reflector 

Tungsten  0.65  w.  p.  m.  h.  c.  in  tinted  reflector 

New   color   matching    unit    (with    0.65    w.  p.  m.  h.  c. 

tungsten  lamp) 

Artificial    sunlight     units     (with     0.7    w.  p.  m.  h.  c. 
tungsten  lamp) 


100 
78 
93 
147 
100 
183 
164 
157 
145 
120 

80 
110 


100 

82 

111 

102 

93 

96 

102 

103 

103 

102 

84 
103 


100 

138 

96 

51 

107 

21 

34 

40 

52 

78 

136 
87 


shown  the  results  obtained  with  this  instrument  on 
Hussey's  daylight  arc  and  other  data  of  interest 
comparable  only  in  a  rough  manner.  The  daylight 
arc  examined  was  a  near  approach  to  daylight  as  far 
as  colorimeter  measurements  can  be  trusted,  although 
it  shows  an  excessive  greenish  component.  This 
could  be  easily  remedied. 

Sharp  and  Millar,15  in  1912,  by  means  of  colored 
screens  and  tungsten  lamps,  also  produced  a  daylight 
effect.  About  this  time  several  units,  designed  to 
produce  artificial  daylight,  appeared,  but  no  examina- 
tion of  these  has  been  made  and  no  quantitative  data 
are  to  be  found  regarding  them. 

The  author  16  has  successfully  used  colored  lamps 
combined  with  clear  tungsten  lamps  by  the  additive 
method,  as  illustrated  in  Fig.  104.  Blue,  green,  and 


244  COLOR  AND  ITS  APPLICATIONS 

blue-green  lamps  were  used  with  success  for  pro- 
ducing daylight  effects  in  combination  with  clear 
tungsten  lamps.  A  notable  installation  was  the  light- 
ing of  the  paintings  at  a  large  temporary  art  exhibit 
in  1913,  where  more  than  400  colored  lamps  were  used. 
This  is  perhaps  the  first  large  exhibition  of  paintings 
where  any  attempt  has  been  made  to  produce  a  day- 
light appearance  by  means  of  artificial  light.  In 
order  to  produce  a  practical  method  for  obtaining  a 
light  of  better  color  value  for  lighting  paintings  and 
other  colored  objects,  many  experiments  have  been 
made,11  with  the  result  that,  besides  the  glassware 
already  described,  metal  reflectors  have  been  used 
having  a  tinted  surface  of  such  a  character  as  to  alter 
the  reflected  light  to  a  color  complementary  to  the 
direct  light  from  the  tungsten  lamp.  Obviously  this 
method  results  in  altering  the  distribution  curve  of 
the  reflector,  producing  in  general  a  less  concentrated 
distribution.  This  indicates  that  focusing  and  inten- 
sive reflectors  of  this  character  should  be  used 
instead  of  those  of  extensive  type.  The  results 
obtained  with  tinted  reflectors  show  that  a  very  good 
quality  of  light  is  obtained  at  a  loss  of  about  50  per 
cent  of  the  original  useful  light.  With  coatings  of 
less  depth  of  color  the  loss  of  light  is  less,  but  the 
improvement  in  quality  is  also  less.  By  changing 
the  shape  of  the  reflector  the  amount  of  the  altered 
light  can  be  varied  within  wide  limits.  For  lighting 
mural  paintings,  for  instance,  the  reflectors  proved 
satisfactory.  No  attempt  has  been  made  to  reproduce 
skylight  or  even  sunlight,  but  a  very  desirable  in- 
crease in  blue  and  blue-green  rays  has  been  obtained, 
as  shown  in  Table  XVII.  The  same  scheme  has 
been  applied  to  the  prismatic  glass  reflector,  a  glass 
coating  being  applied  in  this  case. 


COLOR  IN  LIGHTING  245 

In  1914  Ives  and  Brady9  produced  a  glass  for 
accurate  color-matching  for  use  with  the  Welsbach 
gas  lamp  or  the  tungsten  lamp. 

Other  units  have  been  developed  more  or  less 
approximating  daylight,  but  some  have  not  fulfilled 
the  claims  made  for  them.  There  appear  to  be  two 
fields  for  artificial  daylight  units:  one  where  accurate 
discrimination  of  colors  requires  a  correct  repro- 
duction of  skylight,  and  another  field  where  coarser 
color  work  is  done,  such  as  in  the  paint  shops  and 
lithographing  plants.  Light  approximating  sunlight 
quality  has  been  found  to  fill  the  requirements  in  the 
latter  field. 

The  lighting  of  paintings  is  treated  in  Chapter 
XIII  and  other  colored  lighting  effects  in  Chapter  XII. 

65.  Effects  of  Colored  Surroundings.  —  The  color 
value  of  illuminants  has  been  a  subject  of  consider- 
able discussion  and  investigation  during  recent  years. 
Most  of  the  work  has  been  done  with  colorimeters, 
which,  owing  to  their  limited  power  of  analysis,  furnish 
data  which  are  likewise  limited.  However,  the  light 
that  reaches  the  object  is  ultimately  of  greater  im- 
portance in  lighting.  This  can  be  greatly  altered  by 
selective  reflection  from  surrounding  colored  objects, 
but  the  effect  has  been  a  much  neglected  phase  of 
lighting.  G.  S.  Merrill17  measured  the  color  value 
of  daylight  on  the  working  plane  in  a  room  after  some 
of  the  light  had  been  reflected  from  the  colored  sur- 
roundings. The  interior  measurements  were  made 
on  clear  and  cloudy  days.  They  showed  consid- 
erable alteration  in  the  color  of  outdoor  daylight. 
The  author18  made  a  study  of  this  factor  in  a  minia- 
ture room  lighted  by  means  of  a  tungsten  incan- 
descent lamp  operating  at  7.9  lumens  per  watt,  green, 
yellow,  and  white  wall  papers  in  various  combinations 


246  COLOR  AND  ITS  APPLICATIONS 

on  the  walls  and  ceiling  and  direct  and  indirect 
lighting  systems  having  been  used. 

In  order  to  illustrate  the  possible  color  change  in 
light  due  to  reflection  from  a  colored  surface,  it  is 
possible  to  take  an  actual  case  and  utilize  spectro- 
photometric  data,  but  for  simplicity  we  will  take  a 
hypothetical  case.  Assume  a  light  source  radiating 
equal  amounts  of  monochromatic  red,  green,  and  blue 
light,  and  that  this  source  is  placed  at  the  center  of 
a  hollow  sphere  the  walls  of  which  are  covered  with 
a  perfectly  diffusing  green  paper.  The  colorimetric 
analysis  of  the  illuminant  may  be  expressed  as 

RGB 
100  100  100 

The  reflection  coefficients  of  this  paper  for  the 
particular  illuminant  are  assumed  to  be  in  per  cent, 

RGB 
25.2  47.2  27.6 

The  light  received  by  the  green  paper  in  the  sphere 
is  reflected  an  infinite  number  of  times.  If  the  walls 
of  the  sphere  are  temporarily  assumed  to  be  white 
and  if  N  is  the  reflection  coefficient  of  the  paper, 
then  the  total  light  falling  on  the  walls  will  be 

Q  =  Q'  +  NQ'  +  WQ'  +  N*Q'  +  .  .  .  =^-x     (1) 

where  Q  =  total  light  falling  on  the  walls  and  Q'  = 
direct  light  from  the  light  source  falling  on  the  walls. 
The  color  of  a  paper  is  generally  determined  by 
measuring  the  color  of  the  light  after  it  has  been 
reflected  once  from  the  paper.  It  is  seen  that  a  total 
reflection  coefficient  of  33^%  has  been  assumed  for 
the  green  paper  for  this  particular  illuminant.  The 
coefficient  of  reflection  may  vary  within  wide  limits 


COLOR  IN  LIGHTING  247 

without  any  change  in  the  color  values.  Based  on 
the  foregoing  assumptions  the  reflection  coefficient 
of  this  paper  for  the  monochromatic  red  light  is  25.2% 
of  the  original  100  units;  47.2%  of  the  total  100 
units  of  green  light;  27.6%  of  the  total  100  units  of 
blue  light.  For  this  case  the  total  red,  green,  and 
blue  components  in  the  light  incident  on  the  wall 
paper  after  an  infinite  number  of  reflections  will  be 
respectively, 

CR  =  Q'v  +  NRQ'R  +  N&'*  +  N&'*  +  .  .  .  =    --  (2) 


QG  =  Q'G  +  NGQ'G  +  WG  +  NGQ'G  +  .  .  .  =  j-  (3) 

CB  -  Q'*  +  NBQ'B  +  N&'B  +  N*Q'B  +  '...-  j-^;  (4) 
and 

Q  =  PR  +  QG  +  QB  =  total  light  on  walls  (5) 

Q'  =  Q'R  +  Q'G  +  Q'B  =  total  direct  light  on  walls       (6) 

WR,  NGJ  NB  are  respectively  the  reflection  coefficients 
for  the  monochromatic  red,  green,  and  blue  compo- 
nents of  the  original  illuminant. 

N*QfR>  ^G^G'J  NB(?'B  are  the  color  values  of  the 
wall  paper  as  determined  by  a  tri-color  method  of 
colorimetry  under  the  light,  Q'. 

Computations  yield  the  results  given  in  Table 
XVIII.  On  plotting  these  percentages  (shown  in  the 
column  on  the  right)  in  a  color  triangle,  it  is  shown 
graphically  as  indicated  in  the  table  that  the  reflected 
light  rapidly  approaches  pure  green  by  successive  re- 
flection, but  of  course  the  intensity  rapidly  diminishes* 
as  is  shown  in  Fig.  106.  It  is  also  instructive  to  plot 
the  logarithm  of  the  intensity  against  the  number  of 
the  reflection  which  gives  a  straight  line.  All  three 


248 


COLOR  AND   ITS  APPLICATIONS 


TABLE   XVIII 

Computations  According  to  Equations  (2),  (3),  and  (4),  showing  the  Changes 

produced  in  the  Light  from  a  Special  Source  by  Successive 

Reflections  from  a  Green  Paper 


The  terms  in 
Equations  (2), 
(3)  and  (4) 

Actual  values 

Percentages 

R 

G 

B 

R 

G 

B 

Q' 

100.00 

100.00 

100.00 

33.3 

33.3 

33.3 

NQ' 

25.20 

47.20 

27.60 

25.2 

47.2 

27.6 

N2Q' 

6.35 

22.30 

7.62 

17.5 

61.5 

21.0 

N3Q' 

1.60 

10.45 

2.10 

11.3 

73.9 

14.8 

N4Q' 

0.40 

4.93 

0.58 

6.8 

83.4 

4.8 

N5Q' 

0.10 

2.33 

0.16 

3.8 

90.0 

6.2 

N6Q' 

0.03 

1.10 

0.04 

2.6 

94.0 

3.4 

N7Q' 

0.52 

0.01 

N8Q' 

0.25 

N9Q' 

0.12 

components    decrease    rapidly    in    intensity  with   the 
number  of  reflections,  but  the  green  component  does 


I  Z  3  4  5  6  7 

NUMBER  OF. REFLECTION 

Fig.  106.  —  Illustrating  the  effect  of  multiple  selective  reflections  of  light  from 

a  green  fabric. 

not  decrease  as  rapidly  as  the  others.  In  Fig.  107 
are  shown  the  relative  values  of  the  three  components 
after  various  successive  reflections.  It  will  be  noted 


COLOR  IN  LIGHTING 


249 


that  the  color  of  the  light  approaches  saturated  green 
as  the  number  of  reflections  is  increased.  In  the 
original  paper  various  computations  were  made  which 
relate  to  conditions  of  so-called  indirect  and  direct 
lighting  which  will  not  be  presented  here.  However, 
these  indicate,  as  is  shown  by  actual  measurements 
described  below,  that  the  color  of  the  walls  and 
ceiling  alter  the  color  of  the  light  in  so-called  indirect 
systems  very  much. 


RELATIVE  PERCENTAGE  OF  COM  FOMENTS 

—  r\)(>l-No-i<y>-JCPc0C 
OOOOOOOOOOC 

^  

^ 

^^ 

<*- 

^ 

x" 

X 

X 

/ 

^^ 

=^.^ 

^^^•^ 

-^3 

^ 

^R— 

^=^ 

:=  

)             1                             3456 

MUMBER  OF  REFLECTIONS 

Fig.  107.  —  Showing  the  relative  proportions  of  red,  green  and  blue  components 
in  the  reflected  light  from  a  green  fabric  after  various  successive 
reflections. 

Actual  measurements  were  made  in  a  miniature 
room  illuminated  by  tungsten  light  (7.9  lumens  per 
watt,  1.25  w.p.m.h.c.)  of  the  color  of  the  total  light 
reaching  the  working  plane.  The  room  was  four  feet 
square  and  four  feet  high  and  the  floor  was  assumed 
to  be  the  working  plane.  The  results  are  presented 
in  Table  XIX  reduced  so  that  the  colorimeter  read- 
ings for  the  tungsten  lamp  used  in  the  investigation 
equal  100  for  each  of  the  three  components.  This 
course  is  considered  legitimate  inasmuch  as  only 


250 


COLOR  AND   ITS  APPLICATIONS 


TABLE   XIX 

Colorimeter  Measurements  in  a  Miniature  Room  under 
Various  Conditions  of  Surroundings 


Red 

Green 

Blue 

1.   Tungsten    lamp,    1.25    w.  p.  m.  h.  c.    (7.9    lumens 
per  watt)  

100 

100 

100 

2.   Tungsten   lamp,    0.65    w.  p.  m.  h.  c.     (17    lumens 
per  watt)  

78 

96 

126 

3.   Carbon  lamp,  3.1  w.  p.  m.  h.  c  

116 

104 

80 

4.   Carbon  lamp,  4.0  w.  p.  m.  h.  c. 

129 

101 

70 

5.   Color  of  dull  yellow  wall  paper  
6.   Color  of  dull  green  wall  paper  

131 
104 

115 
119 

54 
77 

(Results  with  tungsten  lamp,  7.9  lumens  per  watt) 
7.  Yellow  walls  and  yellow  ceiling,  indirect  
8.   Yellow  walls  and  yellow  ceiling,  direct  

159 
143 

111 
107 

30 
50 

9.   Yellow  walls  and  white  ceiling,  indirect 

130 

107 

63 

10.   Yellow  walls  and  white  ceiling,  direct  

111 

106 

83 

11.   Green  walls  and  green  ceiling,  indirect  
12.   Green  walls  and  green  ceiling,  direct 

108 
109 

139 
113 

53 

78 

13.   Green  walls  and  yellow  ceiling,  indirect  
14.   Green  walls  and  yellow  ceiling,  direct  
15.   Green  walls  and  white  ceiling,  indirect  .    . 

145 
119 
110 

128 
119 
102 

27 
62 
88 

16.   Green  walls  and  white  ceiling,  direct 

106 

104 

90 

Reduced  Colorimeter 
Readings 


the  relative  magnitudes  of  the  alterations  on  color 
are  desired.  For  the  sake  of  comparison  the  color- 
imeter readings  in  the  same  scale  for  other  incan- 
descent lamps  are  presented.  -  It  is  seen  .'that  .-ordinary 
wall  paper  of  dull  yellow -xolor /may .-alter /the  : color 
of  tungsten  light  so  that  the  useful  light  is  more 
yellow  than  the  old  carbon  incandescent  lamps.  This 
is  a  factor  too  often  neglected,  and  there  are  cases 
where  lighting  experts  have  striven  to  improve  the 
color  of  artificial  light  by  partially  correcting  glass- 
ware, yet  this  light  was  permitted  to  be  largely  re- 
flected from  yellowish  walls  and  ceiling.  In  stores 
and  other  interiors  where  attempts  are  made  to  cor- 
rect the  artificial  light  the  surroundings  should  be 


COLOR  IN  LIGHTING  251 

of  a  neutral  shade  or  of  a  slightly  bluish  tint  if  this 
is  compatible  with  the  color  scheme  of  decoration. 
Many  possibilities  arise  where  the  tinting  of  light  by 
reflection  can  be  utilized,  for,  as  is  seen  by  the  fore- 
going, the  effect  can  be  of  considerable  magnitude. 

65.  Color  in  Interiors. — This  subject  is  largely 
of  interest  to  the  decorator,  and  inasmuch  as  this 
book  is  chiefly  confined  to  the  science  of  color,  the 
aesthetic  side  of  color  will  not  be  considered  except- 
ing in  so  far  as  lighting  aids  the  decorator.  Some  first 
principles  of  interior  decoration,  however,  may  not  be 
out  of  place  here.  A  room  has  been  likened  to  a 
painting:  the  floor  representing  the  foreground;  the 
walls,  the  middle  distance;  and  the  ceiling,  the  sky. 
A  ceiling  may  be  lowered  apparently  by  treating  the 
walls  horizontally,  that  is  by  finishing  the  lower  por-* 
tions  of  the  walls  a  dark  shade  and  the  next  section 
a  lighter  shade  to  within  two  or  three  feet  from  the 
ceiling  and  permitting  the  ceiling  finish  to  extend 
down  the  walls.  Some  decorators  insist  that  color 
has  much  to  do  with  the  apparent  size  of  a  room,  the 
lighter  tints  seemingly  enlarging  the  room. 

The  color  of  a  room  creates  its  atmosphere.  No 
single  color  can  produce  the  best  effect  any  more 
than  one  note  can  produce  a  melody  in  music.  It  is 
the  artistic  variation  in  values  and  tints  that  satisfies 
the  eye.  The  principles  of  masses,  spaces,  and  con- 
trasts, as  well  as  sequences  in  hue  and  brightness,  play 
their  part  in  harmonies  of  color.  The  law  of  appro- 
priateness is  as  important  here  as  in  other  fields,  yet 
color  and  brightness  are  largely  matters  of  individual 
taste,  thus  limiting  the  artist  in  formulating  rules 
which  at  best  are  not  thoroughly  understood. 

North  rooms,  or  those  shielded  from  direct  sun- 
light, are  in  general  more  satisfactory  when  colored 


252  COLOR  AND  ITS  APPLICATIONS 

in  rose,  cream,  yellow,  buff  —  the  'warm'  colors. 
Yellowish  tints  in  the  window  curtains  aid  in  giving 
the  effect  of  sunshine.  On  the  sunny  side,  rooms 
will  perhaps  be  more  satisfactory  when  colored  pale 
blue,  gray-green,  or  shades  and  tints  of  other  'cool' 
colors.  In  introducing  color  into  the  illuminant  by 
means  of  colored  shades  or  lamps  the  color  scheme  of 
the  room  should  be  considered.  Apparently  many 
prefer  bright  red  wall  coverings,  if  one  may  draw 
conclusions  from  observations.  This  again  is  a  matter 
of  personal  taste,  but  extremely  pure  and  bright  colors 
in  lighting  effects  in  interiors  are  to  the  author  like 
living  with  a  brass  band.  Many  of  the  lighting  effects 
in  pure  colors  certainly  arise  from  a  lack  of  study  of 
the  use  and  influence  of  color.  If  a  room  is  decorated 
for  natural  lighting,  theoretically  it  should  receive  the 
same  artificial  lighting  both  as  to  direction  and  spec- 
tral character.  Yet  the  change  in  the  lighting  - 
from  natural  to  artificial  —  may  be  just  the  thing  to 
relieve  monotony.  There  are  many  statements  on 
this  subject  that  cannot  be  reconciled  with  the  facts. 
For  instance,  a  person  may  be  satisfied  with  daylight, 
living  under  it  from  day  to  day  without  any  other 
comment  than  that  it  is  ideal.  The  same  person, 
however,  may  object  to  the  increasing  'whiteness' 
of  modern  artificial  illuminants.  He  insists  that  we 
must  go  back  to  the  color  of  the  carbon  incandescent 
lamp,  or  even  further  to  that  of  the  candle  flame.  Is 
there  a  dual  standard?  Can  daylight  be  satisfactory 
and  the  light  of  the  tungsten  lamp  or  Welsbach  mantle 
be  too  'white'?  As  a  matter  of  fact  all  modern  illu- 
minants used  in  ordinary  interiors  —  the  gas  and 
incandescent  filament  lamps  —  are  in  the  same  class 
and  far  yellower  than  daylight  that  enters  interiors. 
Color  is  certainly  the  keynote  of  lighting  in  many 


COLOR  IN  LIGHTING  253 

interiors,  but  let  us  not  base  its  use  upon  incorrect 
premises.  If  we  prefer  *  warmer'  colors  in  our  arti- 
ficial illuminants,  let  us  have  them,  but  let  us  attribute 
this  desire  to  the  proper  cause,  which  may  be  a  love 
for  change  in  color.  Slight  tints  of  rose  and  yellow 
may  add  something  pleasing  to  the  complexion,  but 
deep  yellow,  orange,  or  red  have  an  obliterating 
effect  upon  the  flesh  tints  of  the  face.  They  also 
tend  to  make  colors  appear  further  from  their  daylight 
appearance  than  untinted  artificial  lights.  Using  color 
for  color's  sake  is  a  legitimate  procedure,  and  in  the 
absence  of  sufficient  physiological  and  psychological 
data  the  use  of  color  must  remain,  for  the  present, 
largely  a  matter  of  taste.  In  lighting  it  is  well  to 
bear  in  mind  the  effect  of  surroundings  in  coloring 
the  useful  light. 

Let  us  take  a  particular  case  —  the  use  of  amber 
glass  with  the  tungsten  lamp  for  aesthetic  purposes. 
A  combination  fixture  had  an  *  indirect'  bowl  from 
which  hung  some  direct  units  with  yellow  silk  shades. 
The  indirect  light  first  passed  through  an  amber  glass, 
then  after  various  reflections  from  ceilings  and  walls 
reached  the  useful  plane.  Inasmuch  as  the  majority 
of  living  rooms  have  wall  coverings  tending  toward 
the  yellow,  brown,  buff,  or  so-called  'warm'  colors, 
the  indirect  component  is  likely  to*  be  considerably 
altered  toward  yellow  in  one  of  these  rooms  without 
the  use  of  amber  glass.  If  the  wall  coverings  are 
of  a  'colder'  tint  why  are  they  satisfactory  under 
daylight  and  not  under  the  far  yellower  artificial  light? 
The  result  obtained  with  the  amber  glass  would  have 
been  obtained  without  it  by  the  use  of  a  more  yellow- 
ish wall  and  ceiling  coverings.  The  color  of  the 
surroundings  depends  upon  the  spectral  character  of 
the  illuminant.  A  yellowish  paper  may  appear  the 


254 


COLOR  AND  ITS  APPLICATIONS 


same  under  a  deep  yellow  light  as  a  yellower  paper 
under  a  pale  yellow  light.  The  object  of  these  re- 
marks is  to  illustrate  that  there  is  some  scientific  or 
physical  basis  for  discussing  any  alteration  of  the 
color  of  artificial  light  tending  away  from  daylight 
in  color. 

Inasmuch  as  amber  glass  is  often  used  as  in  the 
foregoing,  it  is  of  interest  to  analyze  it.     The  author 


100 
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Fig.  108.  —  Screen  for  altering  tungsten  light  to  the  same  spectral  character 
as  carbon  incandescent  electric  light,  c,  d,  e  show  the  transmission 
curves  of  amber  glasses  of  different  densities. 

has  considered  it  unsatisfactory  for  the  above  pur- 
pose because  of  its  greenish  tinge,  and  has  therefore 
sought  for  a  yellowish  glass  or  dye  without  this 
greenish  tint.  Inasmuch  as  amber  glass  is  usually 
used  for  the  purpose  of  altering  the  present  illumi- 
nants  to  a  color  approximating  the  yellow  light  of  the 
carbon  filament  lamp,  kerosene  or  candle  flame,  let 
us  take  the  case  of  altering  the  light  of  a  tungsten 
lamp  operating  at  7.9  lumens  per  watt  to  the  color 
of  the  old  carbon  lamp  operating  at  4  w.p.m.h.c.  This 


COLOR  IN   LIGHTING 


255 


can  be  done  at  a  loss  of  not  more  than  20%  of  the 
total  light;  that  is,  the  tungsten  lamp  operating  at 
1.25  w.p.m.h.c.  will,  with  a  yellow  screen,  produce 
light  closely  approaching  that  of  the  above  carbon 
lamp  at  about  1.5  w.p.m.h.c.  Thus  light  similar  in 
color  to  carbon  incandescent  lamp  light  can  be  ob- 
tained at  a  high  efficiency  with  the  tungsten  lamp. 

Curve    a,    Fig.    108,    represents    the    transmission 
curve   of   an   ideal   screen   for   altering   the   tungsten 


120 
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Fig.  109.  —  Comparison  of  ideal  screen  a,  Fig.  108,  with  amber  glass. 

light  to  a  spectral  character  close  to  that  of  the  old 
inefficient  carbon  lamp.  Curve  c  is  the  transmis- 
sion curve  of  a  light-density  sample  of  amber  glass. 
Curves  d  and  e  are  the  transmission  curves  of  thicker 
specimens  of  the  same  amber  glass.  Curve  d  is  the 
result  of  reducing  curve  a  at  all  points  by  8%.  It 
is  seen  that  the  amber  glass  is  far  from  ideal,  there 
being  an  excessive  transmission  of  green  light  com- 
pared with  the  ideal  curve  a  (or  b).  As  the  density 
or  thickness  of  the  amber  glass  is  increased  the  trans- 
mission of  green  rays  decreases  relatively  more  than 


256  COLOR  AND  ITS  APPLICATIONS 

the  yellow,  orange,  and  red  rays;  that  is,  the  dominant 
hue  shifts  toward  red,  which  is  apparent  by  casual 
observation.  In  Fig.  109  the  transmission  curve  of 
the  ideal  yellow  glass  for  the  above  purpose  is  plotted 
as  a  straight  dashed  line,  and  the  transmissions  of 
light  and  dark  amber  glasses  relative  to  those  of  the 
ideal  screen  are  plotted  as  shown.  It  is  seen  here, 
expressed  analytically,  what  careful  observation  indi- 
cates to  be  true:  that  amber  glass  is  far  from  ideal 
for  altering  modern  illuminants  to  a  color  similar  to 
that  of  the  older  illuminants  which  many  claim  to  be 
the  more  aesthetic. 

By  means  of  present  day  tungsten  lamps  used  with 
a  proper  colored  screen  the  light  of  the  kerosene 
flame  can  be  closely  imitated  at  efficiencies  from  5  to 
10  lumens  per  watt  depending  upon  the  efficiency  of 
the  unaltered  source  employed.  The  light  of  the  old 
carbon  incandescent  lamp  can  be  imitated  in  the  same 
manner  at  efficiencies  from  7  to  13  lumens  per  watt. 
The  author  has  treated  this  subject  elsewhere.24 

Screens  for  this  purpose  are  readily  made  by  the 
use  of  dyes,  although  they  will  in  general  lack  per- 
manency. Most  yellow  dyes  are  objectionable  for 
the  above  purpose  for  the  same  reason  as  amber 
glass.  Potassium  bichromate  is,  under  moderate 
conditions,  a  permanent  yellow.  To  this  may  be 
added  a  pink  dye,  which  will  usually  yield  a  com- 
bination which  is  satisfactorily  yellow.  For  the  pink 
some  very  dilute  red  dyes  may  be  used.  Rhodamine 
is  satisfactory  in  color,  but  is  very  fugitive  under  the 
influence  of  light  and  heat.  Many  yellow  dyes  are 
quite  permanent  if  used  on  a  sheet  of  glass  instead 
of  directly  on  the  lamp  bulb,  but  usually  these  must 
be  corrected  as  already  indicated. 

Much  pleasure  can  be  derived  from  the  use  of 


COLOR  IN   LIGHTING  257 

tinted  illuminants,  for  they  lend  themselves  to  deco- 
rative effects  and  afford  an  easy  means  for  eliminat- 
ing monotony  in  lighting.  Two-  or  three-circuit 
pendant  units  (preferably  indirect  or  semi-indirect) 
are  convenient  for  this  purpose,  for  by  using  clear  and 
colored  lamps  various  combinations  of  color  and  in- 
tensity can  be  obtained  which  are  very  pleasing. 
Silk  shades  of  various  tints  are  readily  applied  to 
lamps,  and  colored  gelatines  are  easily  concealed  and 
afford  a  ready  means  of  obtaining  pleasing  colors. 
(Colored  media  are  discussed  in  the  last  chapter.) 
Very  brief  descriptions  of  a  few  uses  of  colored  light 
in  interiors  may  aid  in  showing  the  possibilities  of 
such  application  of  the  art  and  science  of  color.  There 
are  many  indications  that  we  are  at  the  beginning 
of  an  age  of  color  appreciation.  It  has  already  as- 
serted itself  in  modern  painting;  and  the  gorgeous 
display  of  color  that  greets  the  visitor  to  such  mag- 
nificent architectural  structures  as  the  Congressional 
Library  in  Washington  and  the  Allegheny  County 
Soldiers'  Memorial  at  Pittsburgh  indicates  that  this 
century  is  likely  to  witness  a  renaissance  in  the 
use  of  colors  in  decoration.  Color  was  the  keynote 
in  the  plans  of  the  Panama-Pacific  Exposition 19 
much  of  which  is  obtained  by  lighting  effects.  Col- 
ored jewels  reflecting  millions  of  images  of  light 
sources,  colored  flames,  moving  color  filters,  and 
lights  of  various  colors  were  woven  into  a  gorgeous 
spectacle.  W.  A.  D'a  Ryan,  who  planned  the  color 
effects,  has  also  used  the  '  scintillator '  with  con- 
siderable success.  Powerful  searchlights  arranged 
to  point  radially  upward  illuminate  clouds  of  steam 
in  various  colors.  The  beams  diverge  from  each 
other  in  a  fan-like  manner.  The  possibilities  in 
spectacular  lighting  are  manifold. 


258  COLOR  AND  ITS  APPLICATIONS 

A  notable  use  of  colored  illuminants  is  found  in 
the  Allegheny  County  Soldiers'  Memorial.  In  this 
splendid  lighting  installation,  which  was  designed  by 
Bassett  Jones,20  mercury  arc  lamps,  tungsten  incan- 
descent lamps,  Moore  tubes,  and  yellow  flaming  arcs 
were  used.  The  ceiling  of  the  auditorium,  which  is 
sixty  feet  above  the  floor,  is  composed  largely  of  glass 
in  decorative  panels.  The  central  panel  is  outlined 
by  means  of  the  pinkish  light  of  the  nitrogen  tube. 
Over  the  corner  panels  yellow  flame  arcs  are  hung, 
and  their  flicker  adds  charm  to  the  colored  ceiling 
which  would  not  be  present  with  perfectly  steady 
light  sources.  The  outer  panels  are  lighted  by  the 
bluish  light  of  mercury  arc  lamps,  and  tungsten  lamps 
stud  the  ceiling,  adding  a  touch  of  brilliancy.  The 
contrasting  of  colors  is  so  harmoniously  accomplished 
that  the  result  is  exceedingly  artistic.  Thus  the 
beauty  of  this  monument  of  decorative  art  is  visible 
at  night  as  well  as  by  daylight,  which  is  too  often  not 
the  case.  There  are  many  other  interesting  applica- 
tions of  color  which  make  this  beautiful  work  of  art  a 
worthy  mecca  for  those  interested  in  color  and  lighting. 

Art  galleries  offer  excellent  opportunities  for  in- 
troducing the  science  of  color  lighting.  As  already 
mentioned,  more  than  four  hundred  colored  tungsten 
lamps  were  used  with  clear  tungsten  lamps  in  cor- 
recting the  lighting  of  a  temporary  art  exhibit.  The 
results  were  extremely  encouraging,  inasmuch  as  they 
met  with  the  approval  of  artists  and  critics  alike. 
This  was  perhaps  the  first  notable  attempt  ever  made 
to  furnish  illumination  of  a  daylight  quality  for  light- 
ing paintings.  This  field  offers  a  splendid  oppor- 
tunity for  development,  which  can  readily  be  done 
by  means  of  the  color-correcting  lamps  and  acces- 
sories now  available. 


COLOR  IN   LIGHTING 


Many  artistic  effects  can  be  obtained  by  the  use  of 
colored  light  in  the  home.  A  slight  rose  or  orange  tint 
in  the  light  is  very  pleasing  and  attractive,  although 
the  choice  of  tints  is  of  course  a  matter  of  taste.  A 
rather  interesting  case  is  found  in  a  dining  room  of 
a  pretentious  residence.  A  large  oval  panel  of  dif- 
fusing glass  is  set  into  the  ceiling,  and  behind  this  a 
great  many  red,  green,  and  blue  lamps  of  low  voltage 
are  placed  in  the  approximate  proportions  of  two  red, 
three  green,  and  five  blue  lamps.  The  lamps  of  dif- 
ferent colors  are  controlled  by  means  of  dimmers  set 
in  the  wall,  so  that  by  varying  the  proportions  of  red, 
green,  and  blue  light  various  qualities  of  light  may  be 
obtained  and  also  a  large  range  of  intensities. 

A  person  who  enjoys  color  can  readily  devise 
many  simple  schemes  for  obtaining  tinted  light.  An 
experiment  which  the  author  found  of  interest  was 
the  production  of  an  artificial  moonlight  effect.  A 
high  decorative  window  in  the  living  room  was 
removed  and  placed  in  the  normal  position  of  the 
storm  sash,  thus  providing  space  for  tubular  lamps 
in  reflectors.  The  window  was  covered  on  the  inside 
with  a  cardboard  of  bluish-green  tint  and  in  the  open- 
ing before  the  window,  a  stained  wooden  lattice  was 
placed,  over  which  an  artificial  rambler  rose  was 
twined.  The  lamps,  which  were  tinted  a  light  blue- 
green,  illuminated  the  bluish-green  cardboard,  which 
as  viewed  through  the  foliage  produced  a  charming 
effect  of  moonlight.  As  the  space  was  narrow  the 
cardboard  was  not  uniformly  bright,  owing  to  the 
proximity  of  the  lamp,  but  this  defect  was  readily 
overcome  by  stippling  the  surface  with  a  black  water- 
color.  Such  effects  are  readily  applied  to  bay  win- 
dows and  other  convenient  places. 

Many    possibilities    present    themselves    to    those 


260  COLOR  AND   ITS  APPLICATIONS 

interested  in  color  lighting.  The  many  colored  media 
available  and  the  diversity  of  the  color  of  commercial 
illuminants  provide  the  means  for  carrying  out  many 
ideas.  Electrically  excited  gases,  such  as  carbon 
dioxide,  neon,  helium  and  mercury  vapor,  contained 
in  glass  tubes,  are  commercial  possibilities  which  have 
not  yet  been  applied  to  the  fullest  advantage  for 
elaborate  colored  effects.  In  the  average  case,  how- 
ever, requirements  are  readily  fulfilled  by  means  of 
ordinary  light  sources  and  colored  media. 

66.  Color  Preference. — It  may  be  of  interest  here 
to  record  the  results  of  some  simple  experiments,  in- 
asmuch as  such  data  may  indicate  eventually  the 
effect  of  the  illuminant  upon  our  preference  for  certain 
colors  and  may  throw  some  light  upon  the  relation 
of  lighting  to  the  pleasing  effect  of  colors.  The  ex- 
periments represent  the  beginning  of  an  investiga- 
tion begun  with  an  object  in  view  which  is  discussed 
in  Chapter  XV  but  are  described  here  as  a  matter  of 
interest.  The  Zimmerman  colored  papers  were  used, 
but  as  there  was  no  saturated  green  paper  one  was 
dyed  and  placed  in  the  series.  This  is  designated 
as  g,  the  other  letters  indicating  the  catalogue  desig- 
nation of  the  various  colored  papers.  Fifteen  col- 
ored papers,  each  four  inches  square,  were  spread 
out  haphazardly  upon  a  white  surface,  the  individual 
papers  being  from  six  to  ten  inches  apart.  The  ob- 
server was  asked  to  study  the  colors  and  pick  them 
out  in  the  order  of  his  preference.  He  was  asked 
to  isolate  the  individual  colors  from  everything  as  far 
as  possible,  choosing  the  color  for  color's  sake  alone. 
In  other  words,  if  possible  he  was  not  to  associate 
the  colors  with  wearing  apparel  or  anything  else.  The 
experiments  were  carried  out  under  ordinary  tungsten 
light  (7.9  lumens  per  watt)  and  also  under  daylight 


COLOR  IN   LIGHTING 


261 


entering  the  window,  in  the  latter  case  no  direct  sun- 
light being  present.  The  intensity  of  illumination  in 
each  case  was  of  such  value  as  would  be  considered 
sufficiently  high  for  viewing  saturated  colors.  The 
two  observations  were  carried  out  at  least  a  week 
apart  and  usually  several  weeks  intervened.  The 
general  consistency  of  the  preference  orders  of  the 


V 


abcdefg      h      kiqnopm 
DESIGNATION  OF  COLORED  PAPERS 


Fig.  110.  —  Showing  the  preference  or  rank  of  a  number  of  fairly  saturated  colors. 

fifteen  observers  was  somewhat  surprising.  The 
mean  results  are  plotted  in  Fig.  110,  the  ordinates 
representing  color  preference.  There  may  be  some 
question  regarding  the  legitimacy  of  the  definition  of 
color  preference,  but  the  procedure  adopted  here  pro- 
vides a  simple  means  of  plotting  the  data.  There 
being  fifteen  colored  papers,  the  least  preferred 
would  be  placed  last  and  ranked  fifteen,  the  highest 


262  COLOR  AND   ITS  APPLICATIONS 

preference  therefore  being  unity.  It  is  seen  that 
the  least  preferred  colors  were  those  of  highest  lumi- 
nosity and  in  general  of  lowest  saturation.  That  is, 
purples  and  highly  saturated  colors  having  hues  cor- 
responding to  the  regions  near  the  ends  of  the  visible 
spectrum,  namely  blue  and  red,  were  definitely  fa- 
vored. This  confirms  a  conclusion  previously  arrived 
at  from  other  observations. 

According  to  E.  B.  Titchener21  there  are  two  types 
of  observers:  one  type  prefers  the  saturated  colors 
and  the  other  definitely  prefers  unsaturated  or  'ar- 
tistic' colors,  but  the  former  type  constitute  a  majority. 
The  author's  observations  indicate  that,  when  colors 
are  chosen  for  'color's  sake'  alone,  the  saturated 
colors  are  almost  invariably  chosen.  E.  J.  G.  Brad- 
ford,22 in  experimenting  with  twenty-six  university 
students  with  a  set  of  fifteen  papers  each  about  30 
inches  square,  found  that  saturated  colors  were  most 
preferred.  He  also  found  that  the  admixture  of  a 
small  proportion  of  another  color  lowered  the  posi- 
tion of  the  color  in  the  preference  order.  Cohn23 
has  also  contended  that  increase  of  saturation  tended 
to  make  a  color  more  pleasing.  Bradford  found  that 
the  order  of  preference  remained  reasonably  con- 
stant by  performing  the  same  experiments  on  three 
observers  after  an  interval  of  two  weeks  and  again 
after  a  lapse  of  twelve  months.  The  subject  of 
color  preference  will  be  treated  further  in  Chapter 
XV,  but  it  may  be  of  interest  here  to  compare  the 
results  obtained  by  Bradford  with  those  obtained  by 
the  author.  In  the  latter's  experiments  nearly  all 
the  colors  were  as  saturated  as  possible,  while  only  the 
first  eight  of  Bradford's  were  'pure.'  Bradford  does 
not  state  the  character  of  the  illuminant  used,  but 
presumably  it  was  daylight,  so  the  daylight  preference 


COLOR  IN   LIGHTING 


263 


order  taken  from  Fig.  110  is  used  for  comparison  in 
Table  XX. 

TABLE   XX 
Color  Preference 


Rank 

Bradford 

Luckiesh 

1. 

Dark  blue 

Dark  blue 

2. 

Saturated  green 

Blue 

3. 

Chocolate-brown 

Red-purple 

4. 

Pale  blue 

Green 

5. 

Slate  blue  gray 

Violet-purple 

6. 

Saturated  crimson 

Deep  red 

7. 

Pale  green 

Orange-red 

8. 

Coffee-brown 

Crimson 

9. 

Bluish  green 

Dull  yellow-green 

10. 

Ink-red 

Orange 

11. 

Cinnamon-brown 

Orange-yellow 

12. 

Pale  pinkish  brown 

Dull  green 

13. 

Bluish  green 

Slate  blue  gray 

14. 

Pink 

Yellow 

15. 

Yellowish  Green 

Lemon-yellow 

A  word  of  caution  is  necessary  regarding  drawing 
conclusions  from  Table  XX.  The  colors  are  de- 
scribed so  indefinitely  and  the  two  series  of  colors 
differed  very  much.  In  one  series  practically  all 
colors  were  as  saturated  as  it  is  possible  to  obtain 
them  by  means  of  pigments,  but  in  the  other  series 
about  half  of  the  colors  were  tints  and  shades.  For 
instance,  in  the  latter  series  chocolate-brown  is  a 
saturated  red  of  a  dark  shade.  Furthermore,  as 
seen  by  Fig.  110,  the  reds  ranked  fairly  high,  but  in 
placing  them  in  order,  as  in  Table  XX,  they  are  near 
the  middle  of  the  list  because  several  colors  ranked 
just  above  them.  Notwithstanding  the  foregoing 
there  is  a  similarity  in  the  two  preference  orders. 
Fig.  110  serves  as  an  indication  of  the  similarity  of 
the  preference  order  of  the  various  observers.  For 


264  COLOR  AND   ITS  APPLICATIONS 

instance,  there  being  15  colors  if  every  observer 
placed  h  (lemon-yellow)  last,  its  rank  Would  be  15. 
The  mean  rank  for  h  was  nearly  14,  indicating  that 
nearly  all  the  observers  placed  it  last.  Dark  blue 
was  placed  first  by  most  of  the  observers. 

As  far  as  the  limited  results  indicate,  there  was  no 
general  difference  in  the  preference  orders  under  the 
tungsten  light  and  daylight,  excepting  under  the  former 
illuminant  the  reds  were  definitely  placed  higher  in 
the  preference  order  than  in  daylight.  This  has 
seemed  apparent  from  previous  observations  as  well 
as  the  indication  that  of  a  series  of  saturated  colors 
the  most  saturated  are  usually  the  most  preferred. 
There  is  some  indication  from  other  experiments 
that  the  relatively  few  who  prefer  tints  instead  of 
saturated  colors,  when  asked  to  choose  the  colors 
for  color's  sake  alone,  are  those  that  are  unable  to 
overcome  the  tendency  to  associate  the  colors  with 
other  things.  It  is  just  this  associational  preference 
order  that  is  of  more  interest  in  this  chapter.  That 
is,  in  lighting  there  is  no  doubt  that  tints  are  more 
proper  or  more  aesthetic.  The  data  which  is  dis- 
cussed in  Chapter  XV  from  the  viewpoint  for  which 
they  were  obtained  are  inserted  here  merely  to  illustrate 
some  points  in  the  matter  of  color  preference.  The 
data  on  this  subject  are  rare  and  the  danger  of  draw- 
ing definite  conclusions  at  the  present  time  is  clearly 
recognized. 

Observation  during  the  past  few  years  has  led 
the  author  to  conclude  that  in  the  matter  of  color 
preference  for  color's  sake  alone,  the  colors  near  the 
ends  of  the  spectrum  and  the  purple  series  are  in 
general  favored.  Artificial  illuminants  are  usually 
poverty-stricken  in  blue  and  violet  rays.  Therefore 
these  colors  can  probably  be  made  to  app.ear  more 


COLOR  IN   LIGHTING  265 

attractive  by  means  of  an  illuminant  having  more  blue 
and  violet  rays  and  less  red  and  orange  rays  than 
ordinary  artificial  light.  Strictly,  the  artificial  day- 
light already  described  is  in  general  the  correct 
artificial  illuminant,  but  experiments  indicate  that,  in 
the  illumination  of  colors  for  pure  decoration,  a  c  white ' 
light  in  which  violet  and  red  rays  predominate  pro- 
duces very  pleasing  results.  A  glass  of  this  char- 
acter was  made  of  a  proper  density  so  that  white 
objects  had  the  same  white  appearance  as  under 
natural  skylight,  yet  such  color  as  the  pinks,  purples, 
blues,  violets,  deep  reds,  appeared  richer.  Inasmuch 
as  in  the  decorative  use  of  color,  exactness  in  hue 
is  not  usually  essential,  and  as  the  color  is  employed 
for  our  pleasure,  it  is  legitimate  to  use  the  illuminant 
that  produces  the  most  pleasing  result.  It  is  well  to 
have  white  objects  appear  white,  yet  if  those  colors 
which  please  us  most  can  be  made  more  pleasing  by 
the  use  of  '  white '  light  of  such  a  spectral  character 
as  described  above,  it  is  within  the  province  of  the 
lighting  expert  to  use  such  a  light.  Cobalt-blue 
glass,  in  the  absence  of  a  specially  made  glass,  will 
produce  these  results  fairly  well  if  chosen  of  proper 
density.  An  ultramarine  blue  screen  used  with  ordi- 
nary artificial  light  will  produce  an  extreme  *  white' 
light  of  this  character.  Prussian  blue  added  to  it 
forms  a  satisfactory  screen  for  this  purpose.  In 
prescribing  such  an  illuminant  one  is  not  committed 
to  the  opinion  that  the  pigments  used  in  ordinary 
decoration  are  not  'rich'  enough  to  begin  with.  Such 
handicaps  are  not  uncommon  in  many  of  the  arts 
employing  color,  and  furthermore  colored  decorations 
are  often  dimmed  by  exposure.  In  any  event  the 
matter  is  one  that  will  be  governed  largely  by  taste 
and  the  adoption  of  such  a  lighting  procedure  as 


266  COLOR  AND   ITS  APPLICATIONS 

indicated  above  is  legitimate  if  it  pleases  those 
concerned. 

The  foregoing  experiments  are  not  described  here 
with  the  intention  of  suggesting  that  saturated  colors 
should  be  used  in  lighting.  They  could  not  be  used 
without  endangering  the  appearance  of  many  colors. 
These  various  comments  have  been  made  with  the 
object  of  suggesting  fields  for  thought  and  experi- 
menting. Of  course  it  is  realized  that  the  matter  of 
color  preference  is  exceedingly  complicated  by  all 
the  phenomena  of  color  vision  and  environment,  yet 
the  foregoing  experiments  are  instructive  if  the  limi- 
tations of  the  results  are  recognized. 

67.  A  Demonstration  Booth.  —  The  most  effect- 
ive manner  of  studying  and  demonstrating  lighting 
effects  is  found  in  the  use  of  a  booth  specially  de- 
signed for  the  purpose.  Having  employed  such  a 
booth  for  several  years  very  successfully  it  appears 
of  interest  to  describe  one  in  detail.  A  number  of 
different  types  have  been  constructed,  but  the  one 
described  here  has  been  most  successful.  In  Fig. 
Ill  is  shown  the  wiring  diagram  covering  the  prin- 
cipal features.  The  dotted  line  enclosing  a  rec- 
tangular space  represents  the  front  dimensions  of 
the  booth,  the  center  being  represented  by  the  mal- 
tese  cross.  The  lamps  represented  by  the  larger 
circles  are  placed  in  their  relative  positions.  Fourteen 
clear  40-watt  tungsten  lamps  indicated  by  numbers 
were  spaced  as  shown  around  the  inside  of  the  box 
near  the  front  side,  thus  providing  light  from  various 
directions.  These  are  controlled  by  a  contact  arm 
arranged  to  rotate.  The  control  apparatus  is  dia- 
grammatically  shown  at  the  right,  spread  out  for  con- 
venience. These  switches  are  actually  placed  in  a 
small  recess  in  the  right  end  of  the  box,  as  shown  in 


COLOR  IN   LIGHTING 


267 


Fig.  112.  Twelve  snap  switches  are  shown  above 
the  rotating  contactor,  of  which  the  upper  six  control 
clear  lamps  as  indicated  by  the  numbers.  The  middle 
switches  Si  and  S2  in  the  upper  two  rows  control, 
respectively,  the  four  lamps  on  the  left  and  right. 


268 


COLOR  AND   ITS  APPLICATIONS 


These  must  be  special  switches  and  the  wiring  con- 
nections have  been  omitted  for  the  sake  of  simplicity. 
The  clear  lamps  are  very  useful  in  demonstrating 
effects  of  light  and  shade  and  for  showing  the  effect 
of  diluting  colors,  or  decreasing  their  saturation,  for 
which  purpose  a  variable  resistance  is  placed  in  series 
with  the  rotating  contactor.  Two  single-pole  double- 
throw  switches  are  shown  at  the  left  of  the  rotating 


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Fig.  112. —  Showing  dimensions  and  locations  of  lamps  in  the  demonstration 

booth. 

contactor  switch,  which  provide  for  quickly  changing 
the  lighting  from  above  to  below  or  from  the  left  to 
the  right  side.  A  motor  Af,  controlled  by  switch  Mr, 
is  placed  on  an  extension  at  the  back  of  the  booth,  so 
that  its  elongated  shaft  can  be  projected  through  the 
back  side  in  the  center.  (A  motor  operating  on 
direct  or  alternating  current  and  capable  of  rotation 
at  a  very  high  speed  is  desirable.)  On  this  rotating 
shaft  such  experiments  as  those  indicated  in  Figs.  14, 
23,  29,  30,  31  are  readily  performed. 

General  lighting  of  many  colors  can  be  obtained 


COLOR  IN  LIGHTING  269 

on  the  objects  placed  at  the  center  of  the  back 
from  the  rows  of  colored  lamps  —  eight  above  and 
eight  below — by  controlling  the  relative  intensities  of 
the  red,  green,  and  blue  lights  by  the  corresponding 
rheostats  indicated  at  the  right.  The  switches  con- 
trolling these  lights  are  /?,  G,  and  B  in  the  bottom 
row  of  the  twelve  snap-switches  at  the  right. 

The  purity  of  the  primary  colors  is  of  great  im- 
portance. These  can  readily  be  made  satisfactory 
by  any  one  acquainted  with  coloring  materials  and 
the  science  of  color-mixture.  Perhaps  the  easiest 
procedure  in  most  cases  is  to  begin  with  a  set  of  lamp 
colorings.  Gelatine  filters  made  as  described  in 
Chapter  XVI  may  also  be  used;  however,  for  demon- 
stration purposes  colored  lamps  provide  a  more  com- 
pact apparatus,  although  in  the  latter  case  constant 
care  of  the  lamps  is  necessary,  owing  to  the  fading 
of  the  colorings  due  to  heat  and  light.  By  using  the 
rheostats  the  mixing  of  colored  lights  can  be  done  on 
a  white  diffusing  surface  hung  on  the  back  of  the 
booth  at  the  center,  which  provides  a  very  satis- 
factory means  for  the  synthesis  of  colors.  The  effects 
of  quality  of  light  on  colored  objects  can  be  readily 
demonstrated,  and  daylight  effects  can  be  easily 
shown  by  adding  blue-green  light  to  the  clear  tung- 
sten light.  In  fact  practically  any  demonstration 
involving  color-mixture  is  possible  with  such  an 
apparatus. 

'  Single  red  (#')>  green  (GO,  and  blue  (B')  lamps 
controlled  by  corresponding  switches  on  the  right,  are 
placed  as  shown  at  the  angles  of  an  equilateral  tri- 
angle, the  green  lamp  being  placed  at  the  upper 
apex.  Interesting  colored  shadow  demonstrations  are 
easily  shown,  the  shadow  experiment  illustrated  in 
Fig.  28  showing  the  primaries,  complementaries  and 


270  COLOR  AND  ITS  APPLICATIONS 

white  light  having  been  developed  for  use  with  these 
lamps.  Many  of  the  effects  described  in  this  book, 
especially  those  in  Chapter  XII,  have  been  developed 
in  the  booth  just  considered.  Other  electric  circuits 
are  also  used,  but  these  will  readily  occur  to  the 
experimenter.  Two  views  of  the  booth  are  shown  in 
Fig.  112  with  dimensions.  Those  interested  in  such 
a  field  will  find  the  use  of  such  a  booth  exceedingly 
interesting  and  instructive.  A  number  of  booths, 
perhaps  not  as  compact  and  universal,  have  been 
used  by  pioneers  in  the  study  of  color  effects.  Sev- 
eral years  ago  Basset  Jones,  a  pioneer  in  the  art  of 
lighting,  employed  such  an  apparatus  in  very  inter- 
esting demonstrations. 

REFERENCES 

1.  Jour.  Franklin  Inst.  1912,  173,  p.  315.     Phys.  Rev.  26,  p. 
498;  25,  p.  123;  Trans.  I.  E.  S.  1908,  p.  301. 

2.  Wied.  Ann.  d.  Phys.  1894,  53,  p.  807. 

3.  Silleman's  Jour.  1889,  38,  p.  100. 

«  4.  Ann.  d.  Chimie  et  d.  Phys.  Ser.  6,  1889,  20,  p.  480.    Comp. 
Rend.  1889,  109,  p.  493;  112,  p.  1176,  p.  1246. 

5.  Berl.  Berichte,  1877,  p.  104;  1880,  p.  801. 

6.  Trans.  I.  E.  S.  1910,  p.  189. 

7.  Brit.  Assn.  Report,  1900,  p.  631. 

8.  Lon.  Ilium.  Engr.  1912,  5,  p.  79. 

9.  Elec.  World,  1911,  57,  p.  1092;   Lon.  Ilium.  Engr.  1911,  4, 
p.  394.    Lighting  Jour.  (U.  S.),  1913,  1,  p.  131. 

10.  Trans.  I.  E.  S.  1912,  7,  p.  73. 

11.  Trans.  I.  E.  S.  1914,  p.  839;  Elec.  World,  Sept.  19,  1914. 

12.  Trans.  I.  E.  S.  1914,  p.  687. 

13.  Bui.  Bur.  Stds.  1909,  p.  265. 

14.  Trans.  I.  E.  S.  1910,  5,  p.  209. 

15.  Trans.  I.  E.  S.  1912,  7,  p.  57. 

16.  Lighting    Jour.    (U.   S.),  April,   1914;    Lon.    Ilium.    Engr. 
March,  1914. 

17.  Proc.  A.  I.  E.  E.  1910,  p.  1726. 

18.  Trans.  I.  E.  S.  1913,  p.  61. 


COLOR  IN  LIGHTING  271 

19.  N.  E.  L.  A.  Bui.  Feb.  1915,  p.  87;  Elec.  World,  1915,  65, 
p.  391. 

20.  Trans.  I.  E.  S.  1913,  6,  p.  9. 

21.  Experimental  Psychology,  New  York,  1910,  p.  149. 

22.  Jour,  of  Psych.  1913,  24,  p.  545. 

23.  Phil.  Stud.  1900,  15,  p.  279. 

24.  Elec.  Rev.  and  W.  E.  1915,  67,  p.  161. 

By  M.  Luckiesh: 

The  Lighting  Art,  1917. 
Artificial  Light,  1920. 
Lighting  the  Home,  1920. 


CHAPTER   XII 
COLOR  EFFECTS  FOR  THE   STAGE  AND  DISPLAYS 

68.  The  Stage. —It  is  not  the  intention  to  treat 
the  use  of  colored  light  in  stage  and  display  effects 
as  they  have  been  practised  heretofore,  but  to  point 
out  some  interesting  new  possibilities  that  have  been 
developed  by  the  application  of  the  science  of  color. 
By  the  use  of  red,  green,  and  blue  lights  any  desired 
color  effect  may  be  produced,  but  the  purity  of  these 
primary  colors  is  very  important.  Apparently  there 
has  been  little  exact  science  of  color-mixture  applied 
to  the  stage.  It  is  true  that  wonderful  effects  have 
been  produced^  but  it  is  just  as  certain  that  the  pos- 
sibilities in  color  effects  have  scarcely  been  touched 
upon.  The  color  effects  of  today  have  not  passed 
beyond  the  play  of  colored  lights  upon  colored  scenes 
in  a  more  or  less  haphazard  manner,  the  final  effects, 
which  are  often  very  attractive,  being  arrived  at  by 
a  'cut  and  try'  method.  Examination  of  colored 
media  used  for  such  effects  show  that  very  often 
impure  colors  are  used.  In  fact,  satisfactory  com- 
mercial colorings  are  rare,  and  it  is  usually  necessary 
to  alter  them  in  order  to  obtain  colored  lights  of 
satisfactory  purity.  As  already  stated,  only  pure 
primary  colors — red,  green,  and  blue — are  essential 
for  producing  a  large  variety  of  colored  effects. 

Colored  effects  are  based  upon  the  principle  that 
the  appearance  of  colored  objects  depends  largely 
upon  the  spectral  character  of  the  light  which  illumi- 
nates them;  that  is,  the  color  of  an  object  is  not 

272 


EFFECTS  FOR  THE  STAGE  AND  DISPLAYS        273 

inherent  wholly  in  the  object  itself.  Things  are  vis- 
ible only  by  virtue  of  the  light  which  passes  from 
them  to  the  eye.  For  instance,  a  red  fabric  appears 
red  because  it  has  the  property  of  reflecting  only  red 
light.  Obviously,  if  red  rays  are  not  present  in  the 
light  under  which  the  fabric  is  viewed,  it  will  appear 
black.  Colors  can  be  made  to  disappear  on  a  light 
background  if  they  are  sufficiently  pure  and  free 
from  *  black '  by  viewing  them  through  a  glass  of 
proper  color.  Pale  blue  lines  on  white  paper  will 
practically  disappear  under  a  deep  blue  light,  and  red 
pencil  marks  on  white  paper  will  be  invisible  under 


(Red  light)  (Green  light)  (Blue  light) 

Fig.  113.  —  Illustrating  the  effect  of  colored  light  upon  the  appearance  of  six 

colored  papers. 

a  pure  red  light  of  proper  color.  This  principle  has 
been  applied  in  stereoscopic  drawings,  the  picture 
for  one  eye  being  printed  in  blue-green  ink  and  that 
for  the  other  in  red  ink.  On  placing  a  blue-green 
glass  before  one  eye  and  a  red  glass  before  the  other, 
a  stereoscopic  effect  is  produced. 

In  Fig.  113  are  shown  the  relative  brightnesses  of  * 
six  colored  papers  under  red,  green,  and  blue  lights, 
the  colored  papers  being  in  the  same  relative  posi- 
tions in  each  group.  The  photographs  were  made 
through  a  very  accurate  filter  specially  made  for  the 
panchromatic  plate  used  (Fig.  90),  and 'therefore  the 
brightnesses  are  shown  as  nearly  in  true  relation  to 
each  other  as  the  limitations  of  photographic  repro- 


274  COLOR  AND  ITS  APPLICATIONS 

duction  permit.  It  is  interesting  to  note  some  of 
the  changes;  for  example,  the  two  middle  colors  re- 
verse in  brightness  when  respectively  illuminated  by 
red  and  green  (or  blue)  lights. 

Carrying  this  principle  further  the  author 1  has 
developed  some  colored  effects  which  show  promise 
of  application  as  the  applied  science  of  color  becomes 
more  thoroughly  understood  and  as  the  cost  of  pro- 
ducing suitable  colored  light  decreases.  In  making 
these  applications  it  must  be  remembered  that  a 
color  is  only  completely  defined  when  analyzed  into 
the  three  factors,  hue,  saturation,  and  brightness.  For 
the  purpose  of  producing  the  disappearing  effects  to 
be  described,  a  simpler  analysis  can  be  used.  That 
is,  it  is  convenient  here  to  consider  separately  the  hue 
of  the  light  that  the  pigment  reflects  and  the  amount 
it  reflects,  the  first  involving  the  spectral  hue  and 
the  latter  its  brightness  (or  value).  A  group  of  col- 
ored patches  on  a  gray  ground  can  be  made  to  dis- 
appear —  that  is  to  become  indistinguishable  from 
each  other  and  the  background  —  when  the  colored 
patches  are  illuminated  by  light  of  such  a  spectral 
character  that  they  reflect  rays  of  exactly  the  same 
character  and  in  equal  amounts  as  the  background. 
This  condition  will  not  hold  for  another  illuminant; 
therefore,  some  of  the  colored  patches  will  be  dis- 
tinguishable under  another  illuminant.  This  dis- 
appearance can  be  produced  in  another  manner.  By 
using  a  light  of  such  character  that  the  colored  patches 
will  reflect  practically  none  of  it  they  will  disappear 
if  placed  on  a  black  or  dark  gray  background.  Both 
methods  have  been  used  in  developing  these  colored 
effects.  The  success  of  the  scheme  depends  largely 
upon  the  choice  of  pigments  properly  related  to  each 
other  and  to  the  colored  lights  employed.  Pure 


EFFECTS  FOR  THE  STAGE  AND  DISPLAYS        275 

transparent  pigments  are  quite  essential.  In  mixing 
the  colors  it  is  necessary  to  understand  the  principles 
of  color-mixture,  for  in  mixing  pigments  there  is 
always  a  tendency  toward  black  (Fig.  20).  A  large 
supply  of  pure  pigments  is  desirable,  so  that  a  pure 
pigment  may  be  selected  instead  of  obtaining  the 
necessary  hue  by  mixture.  For  example  green  can 
be  made  by  mixing  yellow  and  blue-green.  This 
subtractive  method  often  results  in  a  green  plus  black; 


Fig.  114. — Illustrating  the  changing  of  scenery  by  the  use  of  colored  lights. 

that  is,  a  muddy  green.  If  the  green  can  be  obtained 
directly  as  a  pigment  instead  of  by  this  mixture,  the 
black  component  is  not  present.  Attention  to  these 
finer  points  is  what  distinguishes  the  scientific  colorist 
from  those  who  arrive  at  results  without  heeding  the 
fundamental  principles.  Owing  to  the  confused  state 
of  color  terminology  and  the  indefinite  notation  of 
pigments,  it  is  impossible  to  describe  accurately  how 
these  disappearing  effects  are  produced.  They  in- 
volve the  science  of  color  and  can  be  produced  readily 
if  the  principles  are  thoroughly  understood. 


276 


COLOR  AND  ITS  APPLICATIONS 


The  modern  tendencies  toward  the  use  of  color 
and  color  effects  point  to  great  future  possibilities 
in  the  application  of  the  science  of  color.  Already 
in  some  European  theaters  the  stage  scenery  has 
been  revolutionized,  and  lighting  effects  are  playing 
a  greater  part  in  the  drama  than  heretofore*  The 
experiments  described  below  suggest  the  possibility 


Fig.  116.  —  Illustrating  the  disappearing  effects  produced  on  a  specially  painted 
scene  by  varying  the  color  of  the  illuminant. 


that  rays  of  light,  swift  and  noiseless,  might  take 
the  place  of  some  of  the  present-day  cumbersome 
methods  of  scene-shifting.  Possibilities  are  also  sug- 
gested for  representing  the  supernatural,  heretofore 
unrealized  on  the  stage.  In  Fig.  114  are  shown,  as 
well  as  can  be  represented  in  black  and  white,  two 
appearances  of  a  mountain  scene.  The  mountain 
and  entire  background  can  be  made  to  disappear  at 
will  by  changing  the  color  of  the  illuminant.  The 


EFFECTS  FOR  THE  STAGE  AND  DISPLAYS        277 

appearance  on  the  left  is  that  under  the  ordinary 
yellowish  light  from  tungsten  incandescent  lamps. 
The  other  appearance  is  that  under  an  orange-red 
light.  The  colors  in  the  foreground  are  violets, 
grays,  blues,  greens,  and  touches  of  yellow.  Those 
in  the  background  are  white,  yellow,  orange,  red, 
and  pink.  Lightning  effects  can  be  obtained  by 
flashing  reddish  light  on  the  painting.  No  attention 
was  paid  to  congruity  in  the  use  of  colors,  for  the 
painting  was  designed  merely  to  illustrate  the  pos- 
sibilities of  the  scheme.  Further  striking  effects  can 
be  obtained  by  the  use  of  illuminants  of  other  colors, 
especially  pale  blue-green  light.  Thus  a  scene  can 
be  changed  by  rays  of  light.  It  is  also  possible  to 
make  the  mountain  disappear  and  in  its  place  to  have 
some  other  scene  appear,  for  instance  a  seascape. 

In  Fig.  115  the  first  picture  appears  to  be  a  Jap- 
anesque arrangement  of  foliage.  This  is  the  appear- 
ance under  a  deep  orange-red  light.  Gradually,  by 
introducing  blue  light,  the  figure  appears,  and  on 
adding  green  light  or  clear  light  it  appears  fully  in 
view.  On  extinguishing  the  red  component  in  the 
illumination  the  figure,  and  especially  the  flowing 
robe,  stands  but  in  strong  contrast  and  beautiful  effects 
are  produced  by  changing  gradually  from  blue-green 
to  a  deep  blue.  By  gradually  introducing  orange-red 
light  and  extinguishing  the  other  components  the 
figure  slowly  disappears.  Such  effects  show  the 
possibility  in  scenic  effects  in  fairyland  plays.  \  It 
is  well  to  understand  that  the  photographic  repro- 
ductions just  shown  only  illustrate  the  brightness 
contrast.  In  the  originals  the  contrasts  are  more 
striking,  because  they  are  due  to  differences  in  hue  as 
well  as  in  brightness.  In  fact,  it  is  difficult  to  illus- 
trate in  black  and  white  the  effects  produced  with  this 


278  COLOR  AND  ITS  APPLICATIONS 

particular  subject,  because  in  the  center  illustration 
most  of  the  contrast  is  due  to  differences  in  hue  alone. 

Another  changing  scene  that  was  produced  is  that 
of  a  summer  landscape  gradually  merging  into  a  snowy 
wintry  scene.  By  painting  the  body  and  branches 
of  the  trees  a  gray,  and  covering  these  and  the  ground 
with  a  bluish-green  foliage,  they  appear  in  their 
abundant  garb  of  summer  under  ordinary  light.  By 
changing  the  color  of  the  illuminant  to  a  'cold'  pale 
blue-green  the  summer  foliage  disappears  from  the 
trees  and  from  the  ground,  and  barren  trees  and  a 
ground  covered  with  snow  appears.  These  are  the 
chief  features  of  this  scene.  Of  course  touches  of 
color  added  judiciously  here  and  there  greatly  en- 
hance the  beauty  of  the  scene.  Many  other  effects 
have  been  produced,  but  no  attempt  has  as  yet  been 
made  in  applying  them  on  a  large  scale  in  stage 
scenery.  However,  the  problem  in  the  theater  is 
comparatively  simple  owing  to  the  perfect  control  of 
the  illumination.  Certainly  the  possibilities  of  such 
applications  of  the  science  of  color  are  very  exten- 
sive. Only  the  simpler  ones  have  been  described 
here,  owing  to  the  necessity  for  demonstrating  the 
principle  as  simply  as  possible.  The  more  elaborate 
effects  require  more  perfect  interrelation  of  colors 
and  illuminants.  A  field  not  to  be  overlooked  is 
that  of  legerdemain,  in  which  such  disappearing  and 
changing  effects  should  prove  valuable. 

69.  Displays.  —  The  foregoing  effects  are  also 
applicable  for  advertising  displays.  In  fact,  it  is 
strange  that  colored  light  has  not  been  applied  more 
to  illuminated  signs.  Large  tungsten  lamps  equipped 
with  color  filters  and  operated  on  flashers  should  add 
considerable  to  the  attractiveness  of  ordinary  scenic 
signs.  The  filters  could  be  such  as  to  produce  moon- 


EFFECTS  FOR  THE   STAGE  AND  DISPLAYS        279 

light,  daylight,  and  sunset  effects  upon  a  scene  with 
great  effectiveness.  Colored  lights  pursuing  each  other 
in  waves  around  the  border  of  a  sign  represents 
a  very  simple  application  of  colored  light  in  adding 
movement  to  an  ordinary  illuminated  sign.  It  seems 
that  the  introduction  of  changing  and  disappearing 
effects  on  illuminated  signs  should  become  popular, 
owing  to  the  lower  cost  as  compared  with  the  cost 
of  an  elaborately  wired  sign  studded  with  incandescent 
lamps.  There  is  no  doubt  that  the  latter  signs  are 
visible  at  a  greater  distance,  but  there  are  a  large 
majority  of  signs  that  cannot  be  viewed  from  a 


THE 

SOCIETY 

FOR 

ELECTRICAL 

DEVELOPMENT 

INC. 

NEW  YORK 


THE 

SOCIETY 

FOR 

ELECTRICAL 

PEVELOPMENT 

INC. 
NEW  YORK 


"DO  IT  ELECTRICALLY"  "DO  IT  ELECTRICALLY" 

Fig.  116.  —  Illustrating  a  flashing  sign  produced  by  properly  relating  the  hue 
and  brightness  of  the  pigments  with  the  colors  of  the  illuminant. 

great  distance  owing  to  obstructions.  In  Fig.  116  is 
represented  a  possibility  outrivaling  the  ordinary 
illuminated  sign,  for  actual  disappearing  effects  are 
produced.  The  copy  in  the  first  view  is  in  red,  orange, 
and  pale  yellow.  This  disappears  under  orange-red 
light,  the  whole  surface  appearing  of  a  uniform  tint. 
The  copy  in  the  second  view  is  in  blue-green.  On 
illuminating  the  sign  with  ordinary  tungsten  light 
the  sign  appears  as  in  the  third  view.  By  alternat- 
ing with  this  clear  tungsten  light  an  orange-red 
light  the  copy  shown  in  the  first  view  appears  and 
disappears.  By  alternating  with  the  clear  light  a 
blue-green  light  the  copy  shown  in  the  second  view 


280  COLOR  AND   ITS  APPLICATIONS 

disappears  and  appears.  Thus  various  effects  can 
be  produced.  This  is  a  very  simple  sign,  requiring  a 
most  simple  wiring  scheme.  The  flashing  lettered 
sign  has  also  been  effectively  combined  with  a  scenic 
painting.  Practically  an  endless  variety  of  effects 
can  be  produced  as  rapidly  as  desired.  Many  other 
effects  have  actually  been  produced,  such  as  a  smiling 
and  frowning  face,  a  gesticulating  speaker,  a  waving 
flag,  and  a  rotating  wheel,  by  this  method  of  chang- 
ing the  spectral  character  of  the  illuminant.  These 
have  been  widely  exhibited. 

This  scheme  has  already  been  applied  in  displays. 
The  haphazard  play  of  colored  lights  upon  colored 
patterns  not  designedly  chosen  is  productive  of  catchy 
attractiveness;  however,  the  actual  disappearing  ef- 
fects are  more  striking.  For  window  displays  the 
copy  is  placed  in  a  darkened  recess  resembling  the 
demonstration  booth  described  in  #67  so  that  it  is 
protected  from  extraneous  light.  The  colored  lights 
are  operated  by  flashers  designed  to  bring  about  the 
proper  sequence  of  appearances.  Such  demonstra- 
tions have  been  built  and  successfully  operated. 
Successful  experiments  have  been  carried  out  with  the 
color  effects  appearing  by  transmission  through  trans- 
lucent glass,  in  this  case  the  colored  lights  being 
behind  the  glass.  The  colored  scene  or  pattern  is 
painted  in  transparent  colors  related  as  before  on  the 
back  of  the  glass.  It  is  also  possible  to  project  the 
colored  light  from  a  distance  by  means  of  parabolic 
reflectors,  which  would  be  an  advantage  in  some  cases 
for  out-door  displays. 

From  the  foregoing  simple  illustrations  several 
advantages  in  the  scheme  are  obvious.  Apparent 
motion  is  obtained  without  elaborate  wiring  or  me- 
chanical devices  excepting  the  usual  flasher.  Copy 


EFFECTS  FOR  THE  STAGE  AND  DISPLAYS        281 

can  be  changed  continually  if  desired  or  a  sign  can 
be  repainted  often.  The  greatest  difficulty  lies  in 
the  initial  preparation  of  the  colors  in  proper  rela- 
tion as  to  brightness  and  hue.  In  order  to  produce 
elaborate  effects  it  will  perhaps  be  necessary  to  use 
water  colors  or  carefully  prepared  oil  paints,  protect- 
ing these  with  a  covering  of  weatherproof  varnish. 
The  only  expense  involved  in  the  change  of  copy  is 
in  the  painting  of  it,  for  the  color  scheme  can  always 
be  retained  in  proper  relation  to  the  colored  illumi- 
nants.  A  flashing  sign  of  this  character  is  very 
simple,  and  the  possibilities  of  scenic  effects  are 
greater  than  in  any  other  method,  simplicity  being 
taken  into  consideration.  The  scheme  adds  to  the 
possibilities  of  stage  effects  where  it  can  be  carried 
out  with  ease  and  sometimes  be  employed  to  sup- 
plant the  jarring  interruption  due  to  shifting  scenery. 
Of  course  the  necessity  of  screening  extraneous  light 
if  present  will  be  a  disadvantage  in  the  application 
of  this  scheme  to  out-door  displays,  but  there  are 
.many  places  where  this  will  be  unnecessary,  because 
numerous  bill-board  sites  can  be  found  where  there 
is  little  or  no  scattered  light. 

The  possibilities  of  the  use  of  colored  light  in 
applying  the  science  of  color  to  displays,  advertising 
and  stage  effects,  have  barely  been  touched.  With 
the  increasing  efficiency  of  light  production  the  utiliza- 
tion of  color  in  lighting  effects  will  become  more 
elaborate. 

REFERENCES 

.  1.   Elec.  World,  April,  1914. 
Amer.  Gas.  Inst.  1913. 
Lon.  Ilium.  Engr.  1914,  p.  158. 
International  Studio,  April,  1914. 
Gen.  Elec.  Rev.,  March,  1914,  p.  325. 


CHAPTER  XIII 
COLOR  PHENOMENA  IN  PAINTING 

70.  Visual  Phenomena.  —  The  artist  has  often 
shown  an  antipathy  toward  science,  apparently  under 
the  impression  that  art  goes  further  than  the  mere 
mixture  and  grouping  of  colors  and  shadows  and 
produces  effects  beyond  scientific  explanation.  By  no 
means  is  it  contended  here  that  art  can  be  produced 
by  'rule  of  thumb,'  or  by  scientific  formulae.  Never- 
theless, facts  are  the  basis  of  all  art  and,  while 
scientific  investigation  has  not  yet  revealed  all  its 
hidden  secrets,  scientific  explanations  can  be  pre- 
sented for  many  supposedly  mysterious  effects.  It 
is  proposed  in  this  chapter  to  present  the  results  of 
analyses  and  to  indicate  that  science  has  been  a  great 
aid  to  art,  and  that  it  will  perhaps  render  a  much 
greater  service  in  the  future. 

The  artist  is  in  reality  a  link  between  two  light- 
ings. He  endeavors  with  chisel  or  brush  to  record 
an  expression  of  light.  The  record  is  therefore  an 
expression  of  light.  Inasmuch  as  both  the  original 
scene  and  the  painted  record  make  their  appeal 
through  the  visual  sense,  it  is  well  to  inquire  into  the 
process  of  vision.  Seeing  involves  the  discrimination 
of  differences  in  light,  shade,  and  color.  In  the  ordi- 
nary sense  no  eye  ever  sees  more,  and  no  painting 
however  'soulful'  has  more  for  its  foundation,  than 
differences  in  light,  shade,  and  color.  (In  the  general 
sense  white,  gray,  or  black  are  colors  of  complete 
unsaturation  and  varying  brightness.  It  will  perhaps 

282 


' 


Plate  IV.    Illustrating  the  effect  of  spectral  quality  of  the  illuminant. 
Daylight,  below;  ordinary  artificial  light,  above 

4 

f: 


COLOR  PHENOMENA  IN  PAINTING  283 

be  more  convenient  in  this  chapter  to  use  the  terms 
4  colors'  and  *  values'  but  with  a  clear  understanding 
that  the  term  *  color'  is  here  used  in  a  restricted  sense 
and  that  value  is  in  reality  included  in  the  term  'color' 
as  used  heretofore.  See  Chapter  IV.)  The  funda- 
mentals of  a  painting  therefore  are  colors  and  values. 
It  is  by  grouping  these  elements  that  the  artist  makes 
his  appeal  to  emotional  man.  However,  science  can 
aid  the  artist  by  analyzing  the  influences  which  alter 
these  fundamentals. 

It  took  the  artist  many  years  to  learn  that  the  eye 
is  far  less  perfect  in  definition  than  a  simple  lens 
and  screen.  In  other  words  everything  in  the  whole 
visual  field  is  not  seen  distinctly  at  the  same  time. 
Definition  is  best  at  the  point  of  the  retina  where  the 
optical  axis  of  the  eye  meets  it,  but  outside  of  a  small 
area  surrounding  this  point,  objects  are  not  seen  dis- 
tinctly. Further,  the  eye  sees  only  the  beginning  and 
end  of  an  ax  stroke  and  it  does  not  see  all  the  move- 
ments of  a  galloping  horse  or  splashing  water.  Pho- 
tography was  hailed  by  many  as  being  a  useful  means 
for  recording  a  scene.  But  photography  has  done 
much  to  teach  the  artist  what  he  should  not  paint  - 
and  that  is  the  realistic  picture  recorded  by  the  pho- 
tographic plate.  Thus  it  is  seen  that  material  facts 
are  often  represented  by  artistic  lies;  that  is,  in 
reproducing  a  scene  the  artist  does  not  record  what 
he  knows  to  be  there,  but  rather  what  he  sees.  In- 
stead of  recording  details  over  the  whole  scene,  the 
artist's  task  is  to  paint  what  the  eye  sees  and  in 
addition,  by  a  sort  of  legerdemain,  to  record  in  colors 
and  values,  as  far  as  is  within  his  power,  the  impres- 
sions gained  through  the  other  senses.  Thus  the 
problem  grows  more  complex,  departing  from  the 
physical  and  entering  the  physiological  and  psycho- 


284  COLOR  AND  ITS  APPLICATIONS 

logical  realms.  The  physical  laws  are  comparatively 
well  understood,  but  the  phenomena  underlying  the 
other  fields  are  still  hazy,  owing  to  the  lack  of  suffi- 
cient experimental  data.  In  viewing  a  painting  the 
problem  becomes  still  more  complex,  for  what  the 
observer  sees  in  a  painting  he  must  largely  supply 
himself  through  the  associational  mental  process. 

There  are  many  vague  terms  used  by  artists,  per- 
haps definite  to  those  who  use  them,  but  the  lack  of 
systematic  usage  is  confusing.  It  has  been  seen 
that  the  eye  is  far  from  being  a  perfect  optical  instru- 
ment. One  of  its  faults  is  chromatic  aberration; 
that  is,  an  inability  to  focus  different  colors  at  the 
same  time  (#33).  Naturally  when  viewing  a  group 
of  different  colors  the  eye  is  focused  for  the  brighter 
colors.  The  eye  is  also  constantly  shifting  under 
normal  conditions.  We  are  not  conscious  of  these 
minute  involuntary  movements,  but  this  shifting  surely 
influences  the  appearance  of  paintings.  The  effects 
of  after-images  are  also  of  importance  (#43).  If  one 
views  a  red  line  on  a  green  or  blue  ground,  the  effect 
is  that  of  unrest.  Both  colors  may  not  be  exactly  in 
focus  at  the  same  time,  but  perhaps  of  greater  im- 
portance are  the  effects  of  overlapping  after-images 
caused  by  involuntary  eye-movements,  which  result 
in  a  'lost  edge.'  The  latter  effect  is  sometimes  very 
striking  at  the  horizon  of  a  landscape  painting.  The 
after-image  caused  by  a  green  stimulus  is  an  un- 
saturated  purple  or  pink.  At  the  edge  where  green 
foliage  meets  a  gray  or  pale  blue  sky  a  hazy  pink 
fringe  is  often  seen.  The  eye,  in  shifting  slightly 
up  and  down,  causes  an  overlapping  of  these  after- 
images (approximately  complementary  to  the  original 
stimulus),  thus  forming  a  'lively'  edge.  The  result 
of  an  after-image  sometimes  is  to  alter  the  saturation 


COLOR  PHENOMENA  IN  PAINTING  285 

as  well  as  the  hue  of  a  colored  area.  The  phenom- 
enon of  simultaneous  contrast  (#44)  is  very  influen- 
tial, and  of  course  is  carefully  studied  by  the  artist. 
This  effect  of  one  area  upon  another  is  of  consider- 
able magnitude  under  some  conditions.  Two  adja- 
cent colored  areas  can  mutually  so  influence  each 
other  that  they  each  appear  differently  in  hue,  sat- 
uration, and  brightness  than  if  viewed  separately. 
On  considering  these  influences  and  those  due  to 
intensity  and  spectral  character  of  the  illuminant,  it 
becomes  evident  that  no  color  has  any  definite  and 
fixed  appearance  after  it  is  out  of  the  tube.  These 
are  facts  which  should  be  of  great  interest  to  the 
artist.  Indeed,  the  great  artists  understood  some  of 
these  influences  very  well.  Most  artists  recognize 
many  of  them,  but  in  general  some  of  the  influ- 
ences are  unknown  to  the  vast  majority  of  painters. 
These  various  phenomena  are  treated  elsewhere.  (See 
Plate  III.) 

71.  Lighting. — Light  has  been  termed  the  soul  of 
art.  The  body  of  a  landscape  consists  of  the  material 
things,  but  as  Birge  Harrison  states,  'its  soul  is  the 
spirit  of  light  —  of  sunlight,  of  moonlight,  of  star- 
light —  which  plays  ceaselessly  across  the  face  of  the 
landscape  veiling  it  at  night  in  mystery  and  shadow, 
painting  it  at  dawn  with  the  colors  of  the  pearl-shell, 
and  bathing  it  at  midday  in  a  luminous  glory.'  But 
of  scarcely  less  importance  is  the  lighting  of  the 
painted  record  of  an  expression  of  light.  In  various 
chapters  it  has  been  shown  what  a  great  influence 
the  illuminant  exerts  on  colors  and  values  —  the  very 
essence  of  a  painting;  however,  slight  attention  has 
been  given  to  this  important  factor.1  The  lighting 
artist  should  be  to  art  what  the  musician  is  to  music. 
His  duty  is  to  render  the  color  symphony  as  the 


286 


COLOR  AND  ITS  APPLICATIONS 


composer  intended  it  to  be  rendered.  This  only  can 
be  done  exactly  by  lighting  the  work  both  as  to  dis- 
tribution and  spectral  character  of  light  just  as  it  was 
lighted  when  the  artist  gave  to  it  the  final  touch. 
This  of  course  is  impossible,  but  it  is  easy  to  light  it 
by  an  artificial  daylight  which  will  render  its  appear- 
ance more  nearly  that  which  is  had  when  completed 
by  the  artist,  and  to  overcome  certain  limitations  of 
pigments  by  properly  distributing  the  light.  (The 


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COLORED  PAPERS 

Fig.  117.  —  Showing  the  reflection  coefficients  of  fairly  saturated  colors  for 
daylight  and  tungsten  incandescent  electric  light.     (See  Table  XV.) 

influence  of  the  illuminant  is  seen  in  #42,  68,  and 
Table  XV.)  Not  only  does  the  spectral  character 
of  the  illuminant  alter  the  hue,  but  likewise  the  bright- 
ness or  value.  In  Table  XV  it  is  seen  that  pigments 


COLOR  PHENOMENA  IN  PAINTING  287 

differ  tremendously  in  reflecting  power  or  relative 
brightness  when  illuminated  with  daylight  and  ordi- 
nary artificial  light.  These  data  have  been  plotted  in 
Fig.  117  to  emphasize  this  important  point.  The 
reflection  coefficients  of  the  various  colored  papers 
for  daylight  are  shown  by  the  dashed  line  RD  and 
for  the  light  from  a  (vacuum)  tungsten  lamp  operating 
at  1.2  w.p.m.h.c.  by  the  full  line  RT.  The  ratios  of 
the  reflection  coefficient  under  tungsten  light  to  that 
under  daylight  are  shown  by  the  upper  curve.  It 
will  be  noted  that  those  pigments  which  predomi- 
nantly reflect  red,  orange,  or  yellow  rays  are  con- 
siderably brighter  under  the  tungsten  light  than  under 
daylight,  but  those  pigments  which  predominantly 
reflect  violet,  blue,  and  green  rays  are  brighter  under 
daylight.  The  curve  shows  that  these  ratios  vary 
from  0.69  to  1.57;  that  is,  some  of  these  pigments 
change  in  relative  ' value'  more  than  50  per  cent. 
However  in  painting,  the  relative  'values'  of  adjacent 
and  other  areas  are  of  importance,  and  such  changes 
in  relative  brightness  are  often  as  high  as  100  per 
cent.  For  instance,  assume  that  clouds  are  adjacent 
to  an  area  of  blue  sky  in  a  certain  painting  and  that 
these  pigments  are  represented  respectively  by  e  and 
n.  The  ratio  of  the  brightness  of  the  clouds  to  that 
of  the  sky  is  1.6  when  the  painting  is  illuminated  by 
daylight.  Under  the  light  from  the  tungsten  lamp 
this  ratio  is  2.8  or  nearly  doubled.  In  fact  cases 
have  been  found  where  such  ratios  have  doubled, 
as  will  be  seen  later.  The  hue  changes  are  in  some 
cases  enormous,  but  these  cannot  be  readily  shown 
here.  The  artist  recognizes  the  difficulty  of  painting 
under  artificial  light,  yet  he  apparently  does  not  raise 
his  voice  in  protest  when  his  work  is  illuminated  by 
artificial  light.  In  Fig.  118  are  shown  the  effects  of 


288 


COLOR  AND   ITS  APPLICATIONS 


COLOR  PHENOMENA  IN  PAINTING  289 

different  illuminants  upon  the  values  of  a  frieze 
painted  with  ordinary  water  colors.  The  illuminants 
were  daylight,  ordinary  tungsten  light  (vacuum  in- 
candescent lamp),  and  an  orange-red  light.  In  this 
frieze  the  upper  rectangles  were  alternately  tinted 
a  reddish  purple  and  a  bluish  purple.  The  lotus 
flowers  and  buds  were  tinted  a  pale  blue,  the  stems, 
dark  green,  and  the  alternate  sectors  of  the  lower 
circular  patterns  were  colored  respectively  a  yellow- 
ish orange  and  a  reddish  orange.  The  background 
was  white.  An  extreme  example  is  shown  in  Fig. 
113.  An  example  of  the  difference  in  the  appearance 
of  a  painting  under  natural  and  ordinary  artificial 
light  is  shown  in  Fig.  119  (see  Plate  IV).  A  photom- 
eter was  used  to  measure  the  relative  brightnesses 
of  adjacent  patches  of  pale  blue  sky  and  pale  yellow 
clouds  at  about  the  center  of  the  sky  area  in  this 
picture,  indicated  by  the  circle.  Under  tungsten  light 
the  two  patches  were  of  equal  brightness,  but  under 
daylight  —  the  light  under  which  practically  all  paint- 
ings are  done  —  the  patch  of  sky  was  twice  as  bright 
as  the  adjacent  clouds.  The  filter  used  in  taking 
these  photographs  was  quite  accurate,  so  that  the 
values  are  faithfully  represented.  It  is  seen  that 
the  sky  was  much  brighter  than  the  foreground  when 
the  painting  was  illuminated  by  daylight.  It  is  only 
fair  to  state  that  the  difference  in  foregrounds  is  due 
somewhat  to  the  lack  of  sufficient  range  of  grada- 
tion in  the  photographic  paper.  Note  also  the  relative 
brightnesses  of  the  low-hanging  clouds.  If  a  paint- 
ing will  stand  such  enormous  changes  in  the  relative 
4 values'  of  its  parts  (and  the  accompanying  changes 
in  hue),  it  is  indeed  flexible.  Under  most  artificial 
illuminants  the  hues  in  a  painting  shift  toward  the 
red  as  compared  to  their  appearance  under  daylight 


290 


COLOR  AND   ITS  APPLICATIONS 


•a 


< 


COLOR  PHENOMENA  IN  PAINTING  291 

illumination.  That  is,  a  deep  yellow  appears  orange,  a 
bluish  purple  appears  a  reddish  purple,  blues  and 
violets  approach  gray,  and  the  reds  are  relatively 
brighter.  Accompanying  this  shift  in  hue  is  a  cor- 
responding shift  in  brightnesses  or  values.  That  is, 
yellow,  orange,  and  red  appear  brighter,  and  violet, 
blue,  and  green  appear  relatively  less  bright,  as  shown 
in  Fig. 

The  c^Bribution  of  light  on  a  painting  has  a  great 
influenc^Tpon  the  expression  of  the  painting.     Meas- 
ureme^^show  that  the  range  of  relative  brightnesses 
in  a  laf  scape  is  often  as  high  as  five  hundred  to  one. 
the    brightest    spot    (for   instance,    cumulus 
receiving    direct    sunlight)   are    often    several 
d    times    brighter    than    the    deepest    shadow, 
igments  employed  by  the  artist  will  not  record 
a  physical  contrast.     In  any  landscape  painting 
brightest  spot  is  seldom  more  than  forty  times 
hter   than   the    darkest    spot   when    both   receive 
actically  the   same   amount   of  light   as   is   usually 
pproximately  the   case.     A  white  paper  is  no  more 
an  fifty  times  brighter  than  a  so-called  black  paper. 
In  order  to  overcome  this  handicap  due  to  the  limi- 
tations of  pigments  the  artist  may  resort  to  illusions 
if   possible.     For   instance,  a   highly   illuminated   red 
object  is  not  painted  red  but  an  orange-red,  because 
it  is  true  that  under  intense  illumination  colors  appear 
less  saturated.     Thus,  by  painting  the  highly  illumi- 
nated red   object  an   orange-red,  the   illusion   of  in- 
tense  illumination  is   produced.     A  hot  desert  scene 
is  depicted  in  the  same  manner,  with  the  additional 
illusion   of   short   or   minimal-length   shadows.     Thus 
the   feeling   that  the   sun  is   at  the   zenith  helps  to 
L    produce  the  illusion  of  a  hot  desert  scene. 
L        R.  W.  Wood  performed  an  interesting  experiment 


292  COLOR  AND  ITS  APPLICATIONS 

in  accentuating  contrast  in  a  painting  by  projecting  a 
positive  lantern  slide  image  of  a  painting  upon  the 
original  in  exact  coincidence.  In  this  manner  the 
high  lights  received  relatively  very  much  more  light, 
and  the  shadows  less  light  than  in  the  ordinary  case, 
where  the  painting  is  uniformly  flooded  with  light. 
This  scheme,  though  interesting  and  instructive,  can- 
not be  used  in  practise.  Extensive  experiments  on 
the  effects  of  distribution  of  light  over  paintings  indi- 
cate that  a  proper  distribution  is  a  legitimate  and 
an  effective  aid  to  the  artist  in  bringing  forth  the 
proper  expression  of  a  painting.  In  Fig.  120  are 
shown  some  effects  of  different  distributions  of  light 
on  a  painting,  although  the  limitations  of  the  photo- 
graphic process  prevent  a  very  satisfactory  illustra- 
tion of  these  effects.  It  is  seen,  however,  that  the 
mood  can  be  changed  enormously  by  altering  the 
distribution  of  the  light.  The  scheme  is  difficult  To 
carry  out  in  cases  where  the  wall  space  is  crowdei 
with  paintings,  and  it  is  unfortunate  for  various 
sons  that  such  crowded  conditions  must  exist.  How 
ever,  the  principle  is  easily  applied  to  individual 
paintings,  and  at  the  same  time  a  correction  of  the 
light  to  daylight  quality  can  be  made.  This  has  also 
been  carried  out  in  the  case  of  trough  lighting,  which 
is  often  a  practical  and  convenient  procedure,  because 
most  paintings  have  their  chief  high  lights  in  the  upper 
portion.  The  predominant  light  can  be  directed  from 
a  point  in  the  trough  near  the  middle  of  the  upper 
edge  or  near  one  of  the  corners  of  the  painting,  de- 
pending upon  the  requirements.  This  has  been  found 
very  effective.  The  lighting  of  paintings  depends 
also  upon  the  hanging,  which  is  too  often  done  with 
a  view  toward  keeping  the  bottom  edges  on  a  hori- 
zontal line  instead  of  with  a  view  toward  placing 


COLOR  PHENOMENA  IN  PAINTING 


293 


I 

0> 

I 


294 


COLOR  AND   ITS  APPLICATIONS 


them  in  the  proper  position  for  lighting  and  observ- 
ing them.  The  wall  covering  is  of  importance  and 
should  be  a  dull,  neutral,  diffusing  surface  and  pref- 
erably rather  dark  from  a  lighting  viewpoint.  This 
prevents  undue  annoyance  from  its  image,  as  seen  in 
the  glass  coverings  of  pictures  on  the  opposite  side 
of  the  room.  The  illusion  produced  by  dark  sur- 
roundings is  striking,  and  there  are  many  who  advo- 
cate such  wall  coverings;  however,  others  contend 
that  the  appearance  of  a  gallery  so  hung  is  unaes- 
thetic.  Much  could  be  written  on  the  daylighting 
of  galleries.  There  have  been  some  extensive  studies 
of  the  problem  made  in  various  countries,  but  there 
is  no  general  agreement  so  far  as  quality  of  light  is 
concerned.  Some  advocate  southern  exposure,  others 
a  northern  exposure.  In  general,  artificial  light  is 
more  readily  controlled  than  daylight,  and  therefore 
lends  itself  more  readily  to  obtaining  proper  effects. 

Inasmuch  as  paintings  are  very 
often  poorly  lighted,  a  simple  illus- 
tration of  the  geometrical  principle 
of  lighting  is  shown  in  Fig.  121. 
Every  problem  is  readily  solved 
by  such  methods.  In  the  design 
\  of  the  lighting,  both  natural  and 
'**  artificial,  it  is  well  to  determine 
graphically  the  positions  of  the  light 
sources  and  the  expanse  of  sky- 
light (if  the  latter  be  diffusing), 
so  that  images  of  these  bright 

sources   cannot  be   reflected  from 

Fig.  121.— illustrating  the    the    glazed    surfaces    of    paintings 

optics  of  picture  lighting.  .  ., 

directly  into  the   eyes. 

72.   Pigments.  —  A    number    of    satisfactory    pig- 
ments, among  which   are   vermilion,  indian-red,  and 


COLOR  PHENOMENA  IN  PAINTING  295 

the  ochres,  are  derived  from  minerals;  the  animal 
kingdom  supplies  such  pigments  as  carmine  and  sepia; 
and  a  large  number  of  pigments,  such  as  indigo, 
gamboge,  and  madder,  are  derived  from  the  vegetable 
kingdom.  Many  of  the  aniline  dyes  are  derived  from 
coal  tar.  Many  pigments  are  made  artificially,  such 
as  ultramarine,  cobalt-blue,  zinc-white,  Prussian-blue, 
chrome-green,  and  the  lakes.  The  natural  pigments 
derived  from  minerals  are  prepared  by  calcining  and 
grinding  and  are  purified  by  washing.  For  oil  paint- 
ing these  pigments  are  ground  in  such  vehicles  as 
linseed  or  poppy  oil.  For  water  colors  the  medium 
is  usually  gum  water.  The  latter  fixes  the  pigments 
on  the  surfaces  to  which  they  are  applied  and  serves 
as  a  varnish.  Such  vehicles  should  preferably  be 
colorless,  because,  for  instance,  the  yellow  color  of 
linseed  oil  is  likely  to  impart  a  greenish  tinge  to  pale 
blue  pigments.  Turpentine  is  used  as  a  thinner  for 
oil  paints.  Varnish  is  employed  to  protect  pigments 
from  destructive  agents  usually  present  in  the  atmos- 
phere and  from  marring  by  abrasion.  Oil  varnishes 
are  less  liable  to  crack  than  spirit  varnishes,  and  the 
quality  of  a  varnish  depends  largely  upon  the  resin 
of  which  it  is  composed.  (See  Chapter  XVI.) 

There  are  three  general  classes  of  pigments  used 
in  paintings.  The  pastel  pigments  are  quite  destruct- 
ible. Water  colors  lend  an  airy  delicacy  to  a  paint- 
ing and  are  quite  appropriate  in  some  classes  of  work. 
They  are  difficult  to  use,  owing  to  their  transparency 
and  to  the  change  in  color  that  they  undergo  on 
drying.  Oil  colors,  which  according  to  some  authori- 
ties were  first  used  in  canvas  painting  in  about  the 
year  1400,  are  the  most  durable  —  an  important  and 
necessary  property  of  pigments  for  use  in  painting. 
Many  pigments  are  permanent  under  moderate  illumi- 


296  COLOR  AND   ITS  APPLICATIONS 

nation  when  used  alone,  but  there  is  always  the  danger 
of  interaction  between  pigments  when  mixed.  The 
permanency  of  the  older  paintings  is  no  doubt  due 
in  part  to  the  fact  that  the  palette  was  rather  poverty- 
stricken  many  years  ago.  Today  there  are  several 
hundred  pigments  available,  and  therefore  there  is 
considerable  danger  of  mixing  pigments  that  interact. 
Anyone  who  has  searched  for  pigments  that  are  per- 
manent under  excessive  illumination  and  heat  will 
perhaps  readily  agree  that  the  permanency  of  pigments 
is  only  a  matter  of  degree  and  that  under  severe 
conditions  many  so-called  permanent  pigments  readily 
deteriorate.  Gases  and  coal  dust  in  the  atmosphere 
and  especially  the  products  of  the  combustion  of 
illuminating  gas  are  known  seriously  to  affect  pig- 
ments. Light  has  a  bleaching  action  and  paintings 
often  turn  yellow  when  kept  in  the  dark.  A  simple 
method  of  restoring  paintings  is  to  clean  them  with 
a  cloth  and  set  them  in  the  sun  for  a  day  or  two. 
This  treatment,  however,  is  rather  severe  for  water 
colors  and  modern  lake  colors  and  is  only  satisfactory 
in  some  cases.  Doubtless  tests  are  being  made  con- 
tinually on  the  permanency  of  pigments,  but  there  are 
few  available  data  on  the  subject.  In  general  min- 
eral colors  are  more  stable  than  vegetable  colors. 
Gases,  moisture,  interaction,  heat,  and  light  are  the 
common  causes  of  the  deterioration  of  pigments.  It 
has  been  found  that  most  pigments  are  more  per- 
manent in  vacuo,  protected  from  harmful  gases  and 
moisture.  Some  of  the  results  of  experiments  indi- 
cate that  the  destructive  rays  in  sunlight  are  chiefly 
the  violet  and  ultra-violet  rays ;  that  is,  pigments  have 
been  found  to  deteriorate  practically  as  quickly  under 
blue  glass  as  under  clear  glass.  However,  the  most 
commonly  used  blue  glass,  namely  cobalt-blue,  trans- 


COLOR  PHENOMENA  IN  PAINTING  297 

mits  deep  red  and  infra-red  rays  almost  as  freely  as 

clear  glass,  so  it  is  possible  that  heat  was  responsible 

for  some  of  the  deterioration.     Of  the  great  number 

of  available  pigments  the  following  are  found  to  be 

most  durable  :  — 

Indian-red,  largely  ferric  oxide; 

Venetian  red,  iron  oxide; 

Burnt  sienna,  calcined  raw  sienna; 

Raw  sienna,  a  clay  containing  ferric  hydroxide; 

Yellow  ochre,  hydrated  iron  oxide; 

Emerald-green,    a    mixture    or    compound    of    copper 

arsenate  and  acetate; 

Terra  verte,  a  natural  green  pigment  found  in  Italy; 
Chromium  oxide,  green; 

Cobalt-blue,  usually  a  mixture  of  the  arsenate,  phos- 
phate, or  oxide  of  cobalt  with  alumina; 
Ultramarine    ash,    now    made    from    soda,    sulphur, 
charcoal,  and  kaolin. 

(Pigments  are  far  from  spectral  purity;  that  is, 
they  reflect  light  of  many  wave-lengths.)  Interesting 
data  obtained  by  Abney  are  shown  in  Table  VI  and 
spectrophotometric  analyses  of  a  number  of  these 
pigments,  including  those  just  described  as  being 
quite  permanent,  are  shown  in  Figs.  122  and  123. 
It  is  seen  that  the  mixing  of  colors  is  complicated 
owing  to  the  complexity  of  the  spectral  character  of 
the  light  reflected  by  pigments.  For  the  sake  of 
clearness  it  will  be  noted  that  the  reflection  curve 
of  a  neutral  tint  (white  or  gray)  surface  would  be  a 
straight  line  parallel  to  the  base  line  in  the  last  two 
illustrations.  The  colorist  should  be  somewhat  fa- 
miliar with  the  spectral  characteristics  of  his  pigments, 
because  such  knowledge  is  very  useful  in  mixing  pig- 
ments. ;The  production  of  different  hues  by  mixing 
pigments  is  possible  because  pigment  colors  are  not 


298 


COLOR  AND   ITS  APPLICATIONS 


monochromatic,   that  is,   not  of  spectral  purity.     For 
instance,  suppose  monochromatic  pigments  were  avail- 


a -Ye  I  low  Ochre         e-  Indiqo 
b-  Co bait  Blue  f-  Terre  Verte 

c-Chromous  Oxide   g- French  Ultramarine 
d- Antwerp  Blue        h- Emerald  Green 


044          045         0.52         0.66          0.60          0.64 
sU,f  WAVE  LENGTH 

Fig.  122.  —  Spectral  analyses  of  pigments. 

i  -Mercuric Iodide          x-      ,     •        ,/  ,, 
j  -  Vet-million         m '  Cod  m  turn  Yellow 
•k-  Gamboge  n  "  Indian 

e  -  Indian  Ye  Ho  w    O  -  Carmine 


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Fig.  123.  —  Spectral  analyses  of  pigments. 

able  and  yellow  and  blue  were  chosen  for  mixing. 
Instead  of  obtaining  green  from  the  mixture,  black 


COLOR  PHENOMENA  IN  PAINTING  299 

would  be  obtained,  because  the  light  transmitted  by 
pure  yellow  flakes  would  be  a  monochromatic  yellow 
which  would  not  be  transmitted  by  pure  blue  flakes; 
thus  by  the  combination  no  light  would  be  trans- 
mitted. One  virtue  of  the  poverty-stricken  palette 
is  the  scanty  possibility  of  the  interaction  of  pigments; 
however,  such  a  palette  cannot  be  the  source  of  a 
large  variety  of  highly  luminous  and  pure  colors. 

Where  high  brightness  and  full  saturation  are 
desired  it  is  well  to  avoid  the  production  of  the  de- 
sired hue  by  mixture,  as  far  as  possible.  This  can 


V  B  G  Y  0  R 

WAVE  LENGTH 

Fig.  124.  — Illustrating  the  effect  of  the  amount  of  the  green  components  in  blue 
and  yellow  pigments  on  the  amount  of  '  black '  in  the  mixtures. 

be  illustrated  by  means  of  the  mixture  of  blue  and 
yellow.  When  these  pigments  are  pure  their  mix- 
ture must  result  in  the  production  of  black.  For 
instance,  suppose  the  two  pigments  transmit  light 
rays  respectively  as  shown  diagrammatically  by  Bl 
and  Yi  in  Fig.  124.  Neither  pigment  transmits  green 
rays  nor  does  one  pigment  transmit  any  rays  that  are 
transmitted  by  the  other;  therefore  the  resultant 
transmission  will  be  zero  and  *  black'  results.  If  the 
so-called  blue  and  yellow  pigments  are  less  pure, 
they  may  be  found  to  transmit  some  green  rays. 
These  may  be  represented  diagrammaticaUy  as  B2 
and  Y2  in  Fig.  124.  It  is  seen  that  the  resulting 
mixture  of  these  two  pigments  will  be  a  green  of  rela- 


300 


COLOR  AND   ITS  APPLICATIONS 


tively  low  brightness  corresponding  to  a  green  to 
which  '  black'  pigment  has  been  added.  The  greater 
the  proportions  of  green  rays  transmitted  or  reflected 
by  the  two  pigments,  the  less  ' black'  will  be  present 
in  the  green  resulting  from  their  mixture,  or,  more 
correctly,  the  brighter  the  resultant  mixture  will  be. 
Obviously,  if  the  two  components  are  selected  suc- 
cessively closer  and  closer  to  green,  finally  the  limit- 


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PIGMENTS 

Fig.  125.  —  Diagrammatic  illustration  of  the  results  of  mixing  blue  and  green 
pigments  containing  various  amounts  of  green. 

ing  case  would  be  that  in  which  both  components 
were  green,  G,  and  their  mixture  of  course  would 
produce  green.  The  results  of  such  a  theoretical 
procedure  are  shown  diagrammatically  in  Fig.  125, 
where  the  sum  of  the  green  components  in  the  blue 
and  yellow  pigments  is  assumed  to  vary  from  zero 
to  100  per  cent;  meanwhile  the  amount  of  green  in 
the  mixture  varies  from  zero  to  100  per  cent,  and  the 
amount  of  *  black'  from  100  per  cent  to  zero.  This 
simple  diagram  illustrates  a  very  important  point  in 


COLOR  PHENOMENA  IN  PAINTING  301 

the  mixture  of  pigments.  For  the  reason  that  the 
subtractive  method  of  color-mixture  always  tends 
toward  the  production  of  *  black'  it  is  well  to  have 
available  a  large  number  of  fundamental  pigments 
representing  as  many  hues  as  possible.  This  is 
especially  desirable  in  the  production  of  effects  de- 
scribed in  Chapter  XII.  Before  J;he  advent  of  modern 
art  such  a  stock  of  pigment  was  not  essential  be- 
cause the  tendency  in  the  past  was  not  to  employ 
colors  as  pure  as  those  found  in  common  use  today. 
A  study  of  the  curves  in  Figs.  122  and  123  is  recom- 
mended in  connection  with  the  discussion  presented 
in  connection  with  Figs.  124  and  125  in  order  to 
obtain  an  idea  of  the  relative  brightnesses  of  various 
mixtures.  See  Chapter  XVII.  ^ 

REFERENCES 

1.  M.  Luckiesh,  Light  and  Art,  Lighting  Jour.  (U.  S.),  March,  1913. 

Light  in  Art,  International  Studio,  April,  1914. 

The  Lighting  of  Paintings,  Lond.  Ilium.  Engr.  March,  1914. 
Lighting  Jour.  (U.  S.),  April,  1914. 

The  Importance   of  Direction,  Quality,  and  Distribution    of 

Light,  Proc  Amer  Gas.  Inst.  1913,  8,  part  1,  p.  783. 
Ostwald,  Letters  to  a  Painter. 
C.  Martel,  Materials  Used  in  Painting. 
E.  N.  Vanderpoel,  Color  Problems,  1903 
C.  J.  Jorgensen,  The  Mastery  of  Color,  1906. 


CHAPTER   XIV 
COLOR  MATCHING 

73.  Nearly  all  the  phenomena  influencing  the 
appearance  of  colored  objects  have  been  treated  else- 
where, but  inasmuch  as  color-matching  is  a  special 
art  and  is  also  of  interest  to  everyone  at  times,  a 
summary  of  those  factors  that  influence  the  appear- 
ance of  colors  may  not  be  out  of  place.  The  expert 
colorist  is  fully  aware  of  the  influence  upon  the  ap- 
pearance of  a  color  of  retinal  fatigue  or  after-images, 
surrounding  colors,  difference  in  sensibility  of  various 
parts  of  the  retina,  the  spectral  character  and  intensity 
of  the  illuminant,  the  surface  character  of  the  fabric, 
the  peculiarities  of  dyes,  and  other  factors.  Every- 
one has  encountered  difficulties  in  distinguishing  or 
matching  colors.  For  instance  it  is  difficult  to  dis- 
tinguish some  blues  under  ordinary  artificial  light, 
owing  to  the  relatively  low  intensity  of  these  rays. 
Many  colored  objects  that  have  appeared  pleasing 
in  daylight  are  so  changed  under  artificial  light  as  to 
be  quite  unsatisfactory.  Usually  under  artificial  light 
the  dominant  hue  of  most  colors  shifts  toward  the 
longer  wave-lengths.  For  instance,  some  purples 
will  appear  quite  red  under  artificial  light  and  bluish 
under  daylight  (Fig.  80).  Such  an  example  is  methyl- 
violet. 

The  surface  character  of  a  fabric  plays  an  im- 
portant part  in  the  appearance  of  the  color.  A  col- 
ored fabric  is  ordinarily  seen  by  reflected  light,  the 
light  falling  upon  it  being  robbed  of  some  of  its  rays 

302 


COLOR  MATCHING  303 


by  the  selective  absorption  of  the  dye.  If  the  surface 
is  porous  like  wool,  the  light  can  penetrate  deeply 
and  will  therefore  suffer  more  internal  reflections 
(#  64),  finally  reaching  the  eye  quite  pure  in  color.  The 
degree  of  transparency  of  the  fiber  also  exerts  an 
influence.  It  is  seen  that  such  dichroic  dyes  as 
methyl-violet,  cyanine,  and  dilute  solutions  of  rhoda- 
mine  or  eosine  pink  will  be  very  much  influenced 
by  the  surface  character  and  composition  of  the  fiber. 
Wool  and  silk  fibers  are  transparent,  but  those  of 
cotton  are  not,  hence  light  cannot  penetrate  as  far 
into  the  latter  as  into  silk  or  wool.  Therefore,  when 
these  three  fabrics  are  dyed  in  the  same  solution  of 
a  dichroic  dye  such  as  methyl-violet  the  cotton  will 
appear  bluer  than  the  other  fabrics.  The  finish  of 
the  surface  is  also  of  importance,  because  of  the  re- 
flection of  unchanged  light  which  dilutes  the  colored 
light  of  the  fabric.  Many  aniline  dyes  exhibit  the 
property  of  fluorescence,  which  alters  the  appearance 
of  the  fabric  under  different  illuminants.  A  fabric 
colored  with  such  a  dye  will  appear  differently  at 
grazing  incidence  than  when  viewed  normally  to  the 
surface.  The  actual  distribution  of  light  is  of  im- 
portance for  the  last  two  reasons. 

74.  The  Illuminant.  —  Inasmuch  as  the  appear- 
ance of  a  color  is  so  influenced  by  its  environment, 
the  question  might  be  asked,  Under  what  conditions 
is  its  appearance  considered  standard?  Daylight  has 
always  been  the  accepted  standard,  because  the  arts 
have  developed  under  daylight.  Furthermore  day- 
light of  a  certain  quality  is  considered  white  light; 
that  is,  it  has  no  dominant  hue.  Such  an  illuminant 
is  logically  a  better  standard  than  an  illuminant  which 
of  itself  will  impart  a  definite  hue  to  the  colored  fabric. 
The  color  of  an  illuminant  (the  color  of  a  white  sur- 


304  COLOR  AND   ITS  APPLICATIONS 

face)  is  largely  a  matter  of  judgment  which  is  influ- 
enced by  many  factors,  and,  inasmuch  as  daylight 
is  quite  variable,  there  has  been  a  lack  of  agreement 
as  to  a  standard  daylight.  Some  have  taken  a  white 
mist  as  representing  such  a  standard,  others  have 
insisted  upon  the  adoption  of  clear  noon  sunlight,  and 
some  have  advocated  the  integral  light  from  the  sky 
and  noon  sunlight.  Nevertheless  a  great  many  col- 
orists  have  adopted  north  skylight  for  color-matching. 

The  difference  between  sunlight  and  skylight  is 
demonstrated  on  viewing  objects  in  the  sunlight. 
Colors  receiving  direct  sunlight  appear  *  warmer,'  and 
the  shadows  which  receive  only  light  from  the  blue 
sky  appear  of  colder  hues,  although  this  comparison 
is  not  wholly  justifiable,  because  of  the  difference  in 
intensity.  Of  course  the  relative  intensities  of  sun- 
light and  skylight  vary  considerably,  but  on  a  clear 
day  a  shadow  on  a  white  blotting  paper,  which  re- 
ceives light  from  the  unobstructed  blue  sky,  is  only 
one-fifth  or  one-sixth  as  bright  as  the  portion  of  the 
paper  receiving  both  direct  sunlight  and  skylight. 
The  color  of  daylight  varies  -throughout  the  day  (#  62). 
Many  colorists  favor  the  use  of  daylight  in  the  fore- 
noon, although  the  morning  light  is  often  of  a  pinkish 
tint.  On  cloudy  dark  days  a  purplish  tint  is  often 
quite  noticeable.  Anyone  engaged  in  accurate  color 
discrimination  is  aware  of  the  continual  changes  in  the 
spectral  character  of  daylight.  Smoke  and  dust  also 
alter  daylight  toward  a  reddish  hue,  and  it  is  likely 
that  the  conditions  in  the  upper  stratum  of  the  atmos- 
phere vary  from  time  to  time,  producing  a  variation 
in  the  character  and  amount  of  scattered  sunlight. 
The  influence  of  colored  surroundings  in  altering  the 
color  of  daylight  is  very  important.  Clouds,  adjacent 
buildings,  green  foliage,  and  the  color  of  interior 


COLOR  MATCHING  305 


walls  exert  a  very  noticeable  influence  on  the  color 
of  daylight.  Perhaps  the  most  annoying  feature  of 
daylight  is  its  unreliableness.  In  some  climates  the 
actual  hours  that  a  satisfactory  daylight  is  available 
for  color-matching  are  few.  In  congested  and  smoky 
cities  this  useful  period  is  further  reduced,  so  that 
there  has  always  been  a  demand  for  an  '  artificial 
daylight.'  Satisfactory  units  of  this  kind  are  now 
available  (#62),  since  the  advent  of  highly  efficient 
steady  light  sources  such  as  the  gas-filled  tungsten 
lamp.  It  is  of  course  impractical  to  furnish  an  arti- 
ficial daylight  to  match  the  different  kinds  of  daylight, 
to  which  various  colorists  have  become  accustomed. 
It  is  too  much  to  ask  any  manufacturer  to  supply 
artificial  daylight  which  will  exactly  match  daylight, 
altered  by  reflection  from  adjacent  colored  objects, 
which  will  be  different  in  most  cases.  If,  however,  an 
artificial  daylight  is  available  to  fill  the  demand  of  the 
colorist,  he  should  be  willing  to  compromise  and  give 
the  unit  a  fair  test.  If  it  differs  slightly  from  the 
daylight  to  which  he  is  accustomed,  yet  shows  no 
peculiar  spectral  characteristics,  the  colorist  can 
readily  adapt  himself  to  the  slightly  altered  condi- 
tions. If  the  artificial  daylight  is  composed  of  inde- 
structible color-screens  the  colorist  can  be  assured 
that  he  has  an  invariable  standard  that  will  serve 
him  twenty-four  hours  a  day  —  a  most  desirable  char- 
acteristic. There  is  some  advantage  in  the  produc- 
tion of  a  colored  glass  screen  for  use  with  tungsten 
lamps  because  the  quality  of  light  can  be  varied  be- 
tween sunlight  to  blue  skylight  by  varying  the  tem- 
perature of  the  lamp  filament.  As  was  shown  in 
#  62  the  glass  developed  by  the  author  alters  the  light 
from  the  vacuum  tungsten  lamp  operating  at  7.9 
lumens  per  watt  to  noon  sunlight  quality;  however, 


306  COLOR  AND   ITS  APPLICATIONS 

when  used  with  the  gas-filled  lamp  operating  at  about 
16  lumens  per  watt,  artificial  skylight  is  produced. 
This  is  a  very  convenient  method  of  utilizing  the 
same  glass  to  produce  artificial  daylight  of  different 
kinds.  Spectrophotometric  tests  afford  the  only  thor- 
ough means  of  analyzing  an  artificial  daylight.  Di- 
chroic  dyes  and  some  mixtures  of  aniline  dyes  are 
greatly  influenced  by  the  spectral  character  of  the 
illuminant  and  therefore  afford  a  ready  means  for 
determining  approximately  the  satisfactoriness  of  an 
illuminant  for  color-matching  purposes.  Such  mix- 
tures can  be  readily  made  so  that  fabrics  dyed  with 
them  will  appear  of  the  same  hue  under  a  certain 
illuminant,  yet  under  another  illuminant  they  will 
appear  quite  unlike.  The  two  dyes  used  in  screens 
c  and  dj  Fig.  17,  are  examples  of  this  character. 
Mixtures  that  appear  green  under  daylight  but  quite 
different  under  another  illuminant  can  be  readily 
made  by  mixing  naphthol-yellow  with  acid-violet  and 
an  orange  with  a  deep  bluish  aniline  dye.  Two  blue 
dyes  can  be  readily  made  to  appear  practically  alike 
under  daylight,  one  consisting  of  a  rather  pure  blue 
and  the  other  having  the  common  characteristic  of 
transmitting  deep  red  rays.  Under  an  artificial  illu- 
minant, rich  in  red  rays,  the  latter  will  appear  quite 
reddish  as  compared  to  the  former.  A  weak  solu- 
tion of  erythrosine  or  rhodamine  when  added  to  a 
weak  solution  of  potassium  bichromate  will  produce 
a  yellow  in  artificial  light ;  however,  in  daylight  it  will 
appear  quite  pink.  Such  combinations  can  be  readily 
produced  by  examining  the  spectra  of  the  dyes  and 
by  combining  them  judiciously.  Excellent  dichroic 
dyes  are  methyl-violet  and  cyanine.  These  striking 
instances  of  the  effect  of  the  illuminant  are  well 
known  to  dyers  and  other  colorists. 


COLOR  MATCHING  307 

75.  The  Examination  of  Colors.  —  In  examining 
colors  it  is  well  to  understand  the  peculiarities  of 
vision.  The  fovea  centralis  of  the  retina,  where 
vision  is  most  acute,  is  directly  opposite  the  middle 
of  the  pupillary  aperture.  A  small  area  around  this 
point  has  been  named  the  macula  lutea.  The  center 
of  this  region,  which  is  called  the  *  yellow  spot,'  owing 
to  its  color,  often  manifests  itself  in  the  examination 
of  colors  (#55).  It  apparently  absorbs  blue  rays 
somewhat,  and  its  effect  is  quite  noticeable  on  view- 
ing bright  colors.  The  effect  is  particularly  notice- 
able roughly  outlined  in  after-images  produced  by 
large  bright  colored  areas.  Bright  colors  are  difficult 
to  examine,  owing  to  retinal  fatigue  and  to  the  promi- 
nence of  after-images  and  successive  contrast.  This 
annoyance  can  be  reduced  by  decreasing  the  intensity 
of  illumination  or  by  the  use  of  neutral  tint  glasses. 
These  methods  are  perhaps  questionable,  but  are 
certainly  less  objectionable  than  the  use  of  blue- 
green  glass,  as  is  used  by  some  for  the  examination 
of  bright  red  and  orange  colors.  Paterson1  recom- 
mends the  use  of  a  gelatine  film  dyed  with  malachite 
green  (a  blue-green)  for  the  examination  of  such 
highly  luminous  colors.  A  practically  neutral  tint 
screen  can  be  made  of  a  solution  of  nigrosine  in 
gelatine. 

The  effect  of  simultaneous  contrast  is  often  very 
great,  for  colors  are  apparently  altered  in  hue  and 
brightness  by  the  influence  of  an  adjacent  color 
(Plate  III).  A  black  pattern  on  a  red  ground  will 
appear  of  a  blue-green  tint.  A  white  surrounded  by 
green  will  appear  brighter  and  of  a  pinkish  tint. 
Chevreul2  published  an  extensive  work  on  this 
subject  many  years  ago  which  goes  into  elaborate 
detail  concerning  contrast.  •  In  order  to  examine  the 


308  COLOR  AND  ITS  APPLICATIONS 

color  of  a  thread  or  portion  of  a  variegated  color  pat- 
tern, it  is  well  to  isolate  the  portion  to  be  examined 
by  means  of  a  gray  mask.  The  effect  of  contrast  is 
so  great  that  a  colored  thread  may  appear  rich  and 
pure  in  one  pattern,  yet  quite  dull  amid  other  sur- 
roundings. This  effect  cannot  be  overlooked  without 
inviting  trouble  in  color-matching. 

If  it  were  not  for  the  effects  of  fluorescence,  color- 
matching  glasses  could  be  used  which  have  been 
especially  adapted  to  the  artificial  illuminant.  How- 
ever, as  many  of  the  aniline  dyes  fluoresce  they 
should  receive  light  of  the  standard  daylight  quality 
while  being  examined.  This  would  not  be  the  case 
with  the  combined  use  of  an  artificial  illuminant  and 
correcting  spectacles.  Of  course  the  intensity  of  the 
artificial  light  must  be  several  times  greater  than 
ordinarily  required  for  seeing  in  order  to  compensate 
for  the  unavoidable  absorption  of  the  color-matching 
spectacles.  This  scheme  is  not  new,  for  it  has  been 
practised  in  special  cases  by  many  expert  col- 
orists. 

Colored  fabrics  are  examined  both  by  transmitted 
and  reflected  light.  Colors  are  usually  viewed  by 
reflected  light,  the  change  in  the  color  of  the  incident 
light  being  due  to  selective  absorption.  In  a  loose 
fabric  of  porous  surface  the  light  penetrates  more 
deeply  and  is  colored  by  many  multiple  reflections. 
As  already  stated  silk  and  wool  fibers  are  more  trans- 
parent than  cotton,  and  therefore  permit  a  deeper 
penetration  of  the  light.  This  means  a  greater  num- 
ber of  multiple  reflections,  and,  for  example,  as  in  the 
case  of  a  dichroic  dye,  it  results  in  a  color  corre- 
sponding to  that  which  would  be  obtained  with  a 
cotton  fabric  dyed  in  a  denser  solution  of  this  dye. 
The  luster  of  silk  is  attributed  to  the  smoothness  of 


COLOR  MATCHING  309 

the  fibers.  In  examining  colors  by  reflected  light 
the  distribution  of  the  incident  light  is  of  importance 
inasmuch  as  some  of  the  regularly  reflected  light  is 
but  slightly  changed,  owing  to  the  fact  that  it  does 
not  penetrate  the  fabric.  This  tends  to  dilute  the 
colored  light  and  to  make  it  appear  less  saturated. 
In  installing  artificial  daylight  it  is  well  to  distribute 
the  light  in  a  manner  found  satisfactory  in  day- 
light. 

Many  aniline  dyes  in  solid  form  reflect  light 
complementary  in  color  to  that  which  they  transmit. 
Crystals  of  some  purple  dyes  appear  green  by  reflected 
light.  If  the  crystal  be  ground  into  a  fine  powder, 
the  latter  appears  purple  in  color,  because  the  light 
penetrates  it  and  by  transmission  and  multiple  re- 
flections appears  different  than  by  specular  reflec- 
tion. A  borax  bead  containing  cobalt  may  appear 
almost  black,  but  when  ground  into  a  powder  it  ap- 
pears blue.  Pigments  when  in  a  dense  homogeneous 
mass  are  quite  opaque  and  reflect  light  selectively. 
The  phenomena  of  surface  color  is  intimately  related 
to  the  coefficient  of  absorption  and  the  refractive 
index  of  the  substance.  Inasmuch  as  the  phe- 
nomenon is  not  of  sufficient  importance  to  go  into 
details  regarding  it,  the  reader  is  referred  to  any 
standard  text-book  in  physics  for  an  analysis  of  the 
phenomenon.  Some  fabrics  exhibit  changeable  colors 
owing  to  their  nap,  which  ends  in  a  certain  direction. 
If  it  ends  toward  the  light,  the  latter  penetrates  the 
fabric  to  a  considerable  depth  and  is  deeply  colored 
by  multiple  reflections.  If  the  nap  ends  away  from 
the  direction  of  the  light,  there  is  more  specular  reflec- 
tion and  therefore  less  penetration,  which  results  in 
a  smaller  change  in  color. 

The  fibers  of  a  fabric  may  be  considered  to  hold 


310  COLOR  AND   ITS  APPLICATIONS 

the  dye  in  a  state  of  suspension  or  solution,  and  there- 
fore fabrics  are  sometimes  examined  by  transmitted 
light.  In  a  special  case  of  this  kind  of  examination 
the  fabric  is  held  between  the  eye  and  the  light  and 
is  viewed  at  a  grazing  angle.  This  is  sometimes 
called  the  overhand  method.  By  thus  looking  through 
the  fibers,  hues  can  be  distinguished  that  are  quite 
imperceptible  in  an  examination  by  reflected  light. 
This  method  is  especially  applicable  to  the  examina- 
tion of  colors  of  the  darker  shades.  The  rich  appear- 
ance of  these  dark  colors  viewed  in  this  manner  is 
very  striking. 

The  change  in  color  that  a  dyed  fabric  undergoes 
on  drying  is  of  great  importance  and  often  quite 
annoying.  This  need  not  be  treated  here,  because 
the  novice  will  learn  very  quickly  that  dyes  in  solu- 
tion and  freshly  dyed  fabrics  often  undergo  great 
changes  in  color.  It  is  a  point  to  be  considered  in 
color-matching.  A  great  many  dyes  exhibit  the  prop- 
erty of  fluorescence,  among  which  are  the  eosines, 
phloxine,  rhodamine,  uranine,  fluorescein,  rose  bengal, 
naphthalin-red,  resorcin-blue,  and  chlorophyl.  In 
matching  strongly  fluorescent  colors  it  is  seen  that 
there  is  quite  a  difference  in  hue  between  the  re- 
flected and  transmitted  light.  For  instance,  by 
reflected  light  an  eosine  pink  will  appear  redder  than 
by  transmitted  light  by  the  overhand  method.  This 
is  due  to  the  fact  that  the  reddish  fluorescence  is 
most  predominant  in  the  light  reaching  the  eye  when 
the  fabric  is  examined  by  the  ordinary  method  of 
reflected  light.  By  the  overhand  method  this  fabric 
will  appear  decidedly  bluer,  owing  to  the  fact  that  the 
fluorescent  light  does  not  reach  the  eye  in  appreciable 
amounts.  This  effect  may  readily  be  demonstrated 
by  dyeing  two  fabrics  respectively  by  fluorescent  and 


COLOR  MATCHING  311 


non-fluorescent  dyes  so  that  they  match  by  reflected 
light.  They  will  be  found  to  appear  different  by  the 
overhand  method. 

REFERENCES 

1.  David  Paterson,  Colour  Matching  on  Textiles,  London,  1901. 

2.  M.  E.  Chevreul,  Principles  of  the  Harmony  and  Contrast  of 
Colours. 


CHAPTER   XV 
THE  ART   OF  MOBILE   COLOR 

76.  This  subject  will  be  treated  from  two  view- 
points: first  as  to  the  relation  of  colors  and  sounds, 
and  second,  from  the  viewpoint  of  an  art  of  mobile 
color  independent  of  any  other  art.  The  treatment 
from  the  first  viewpoint  is  not  entirely  one  of  choice. 
In  fact  one  feels  compelled  to  discuss  the  possibility 
and  justification  of  such  a  relation  because  in  the 
few  instances  that  colors  have  been  related  to  sound 
music  the  superficiality  has  been  quite  apparent.  It 
took  centuries  of  scientific  study  and  analysis  to 
mould  musical  chaos  into  a  uniform  art  of  measured 
music,  and  even  today  there  are  composers  who  are 
not  reconciled  to  the  generally  accepted  state  of 
affairs.  Even  with  this  example  of  slow  evolution 
in  sound  music  before  them,  there  have  been  a  few 
who  have  had  the  temerity  to  relate  colors  and  music 
before  the  public  notwithstanding  the  meager  data 
available.  It  is  significant  that  the  names  of  these 
'  inventors '  are  not  found  among  the  experimental 
psychologists  and  other  investigators  who  are  un- 
earthing information  that  may  some  day  form  the 
foundation  of  an  art  of  mobile  color. 

Rimington,  in  a  book  entitled  *  Colour-Music,'  re- 
peatedly compares  colors  and  sounds,  owing  to  the 
fact  that  both  'are  due  to  vibrations  which  stimulate 
the  optic  and  aural  nerve  respectively.'  He  further 
states  that  'this  in  itself  is  remarkable  as  showing 
the  similarity  of  the  action  of  sound  and  color  upon 

312 


THE  ART   OF  MOBILE   COLOR  313 

us.'  He  presents  other  '  similarities '  but  in  fairness  it 
should  be  noted  that  he  states  that  too  much  weight 
should  not  be  given  to  them.  Nevertheless,  owing 
to  the  repeated  citations  by  Rimington  of  these  'simi- 
larities' one  concludes  that  they  influence  him  con- 
siderably in  developing  his  so-called  'color  organ.' 
If  no  stronger  reason  for  interest  in  the  art  of  mobile 
color  existed,  space  would  not  be  given  to  a  discus- 
sion of  this  subject,  but  there  are  indications  that  such 
an  art  is  waiting  to  be  evolved.  Furthermore,  the 
relation  between  sound  and  color  forms  such  an  in- 
significant part  in  the  author's  thoughts  regarding 
color  music,  that  space  would  not  be  given  to  such 
a  discussion  if  it  did  not  appear  necessary  to  clarify 
the  matter  by  dispelling  some  of  the  superficial 
ideas  regarding  such  a  relation  and  by  pointing  out 
the  limitations  of  certain  attempts  to  present  such  a 
combination. 

There  is  no  physical  relation  between  sounds  and 
colors.  Sounds  are  transmitted  by  waves  in  a  mate- 
rial medium,  as  proved  by  many  experiments.  Light 
rays  are  supposed  by  many  to  be  transmitted  by  a 
hypothetical  medium  called  the  ether,  but  scientists 
are  divided  in  their  opinions  regarding  the  existence 
of  an  ether.  Furthermore,  the  two  kinds  of  wave 
motion  that  are  used  to  represent  sound  and  light 
waves  are  necessarily  different,  because  the  former 
cannot  be  polarized  while  the  latter  can  be.  Light 
waves  pass  through  what  we  term  a  vacuum,  but 
sound  waves  cannot.  These  few  fundamental  differ- 
ences are  sufficient  to  illustrate  the  futility  of  any 
claims  that  sounds  and  colors  are  produced  in  similar 
ways. 

Next  let  us  consider  the  respective  perceiving 
organs.  The  ear  is  analytic,  for  a  musical  chord 


314  COLOR  AND   ITS  APPLICATIONS 

can  be  analyzed  into  its  components.  This  is  not 
true  of  the  eye.  In  other  words,  the  eye  is  a  syn- 
thetic instrument  incapable  of  analyzing  a  color  into 
its  components.  Many  examples  have  been  cited 
in  previous  chapters  of  colors  that  appeared  identical 
to  the  eye,  yet  differed  greatly  in  spectral  character. 
This  difference  in  the  two  organs  must  necessarily 
influence  the  choice  of  a  fundamental  mode  of  pro- 
ducing '  color  music.' 

As  already  stated,  it  is  noteworthy  that  those  few 
persons  who  have  actually  written  '  color-music '  are 
not  found  among  the  large  group  contributing  to  the 
development  of  the  science  of  experimental  psychol- 
ogy or  to  sciences  closely  akin  to  it.  The  relation 
between  colors  and  sound  music,  if  any  exists,  some 
day  will  be  revealed,  but  only  through  systematic 
experimentation  by  investigators  well  versed  in  phys- 
ics, physiology,  and  psychology.  There  is  value  in 
experiments  directly  relating  colors  and  music,  but 
certainly  it  is  too  early  to  experiment  before  the 
public.  Such  procedure  jeopardizes  the  chance  for 
ultimate  success,  but,  fortunately,  past  exhibitions  of 
this  character  will  have  been  forgotten  long  before 
color-music  evolves  into  a  form  in  which  it  will  be 
recognized  ultimately. 

For  some  time  the  author  has  been  interested 
in  the  subject  of  mobile  color  as  a  mode  of  expres- 
sion similar  to  the  fine  arts,  and  has  therefore  watched 
with  interest  some  attempts  in  relating  colors  and 
music.  This  interest  has  been  almost  entirely  in  an 
art  of  mobile  color  independent  of  any  other  art,  but, 
besides  preliminary  experiments  bearing  on  the  sub- 
ject, some  experiments  with  colors  and  music  have 
also  been  performed.  These  will  be  touched  upon 
later.  Recently  a  musical  composition  by  A.  Scria- 


THE  ART  OF  MOBILE   COLOR 


315 


bine  entitled  '  Prometheus '  was  rendered  by  a  sym- 
phony orchestra  with  an  accompaniment  of  colors 
according  to  the  'Luce*  part  as  written  by  the  com- 
poser for  the  'Clavier  a  lumieres'  (Fig.  126).  No 
clue  is  found  in  the  musical  score  regarding  the  colors 
represented  by  the  notes  in  the  'Luce'  part,  or  the 


A.SCR1ABINE 

_PROM£THEE_ 

LEPOEMEDUFEU 

POUR  GRAND  OR(mSTRK  ET  PIANO 
AVEC  ORtillK.  CHOHURS 

ETCL'WlLRAlJJMII-RliS 


r^dg* 

. 


Fig.    126.  — The   'Luce'  part  for  the    'Clavier    a  lumieres'  in  Scriabine's 
'  Prometheus.'     (Upper  staff  in  each  portion  is  the  '  Luce '  part.) 

manner  in  which  a  'colored  chord'  is  to  be  played  — 
whether  by  juxtaposition  or  by  superposition.  The 
latter  point  is  of  fundamental  importance,  inasmuch 
as  the  eye  is  not  analytic  and  a  mixture  of  the  colors  of 
a  'color  chord'  results  in  only  a  single  hue.  Some  of 
those  responsible  for  the  rendition  of  this  music,  with 
color  accompaniment,  had,  at  different  times  previous 
to  the  final  presentation,  accepted  both  the  Rimington 
scale  and  Scriabine's  code  (the  latter  having  been 
discovered  later  in  a  musical  journal  published  at 
the  time  of  a  previous  presentation  of  the  same  selec- 


316  COLOR  AND   ITS  APPLICATIONS 

tion  in  London)  as  being  properly  related  to  the 
music.  The  acceptance  of  the  Rimington  scale,  in 
the  absence  of  Scriabine's  code,  as  being  adapted  to 
the  music,  and  the  final  acceptance  of  the  latter  code, 
which  was  used  in  the  public  presentation,  shows 
that  at  the  present  time  there  is  no  definite  relation 
between  colors  and  sound  music,  even  in  the  minds 
of  artistic  interpreters  of  music.  It  must  not  be 
assumed  that  the  colors  in  Table  XXI  bear  any  abso- 
lute relation  to  the  corresponding  musical  notes. 
Rimington's  scale  apparently  was  chosen  arbitrarily, 
as  shown,  merely  for  convenience  in  writing  a  *  color 
score/  This  is  probably  true  of  Scriabine's  scale. 
Those  familiar  with  the  science  of  color  would  hardly 
consider  it  probable  that  a  composer  of  sound  music 
would  hold  the  key  to  '  color  music '  when  they 
freely  acknowledge  their  helplessness  in  definitely 
relating  colors  and  musical  sounds.  Everything 
pointed  to  failure,  and  if  one  may  judge  from 
the  criticisms  of  the  rendition  of  *  Prometheus'  with 
the  accompaniment  of  colors,  after  allowing  for 
a  considerable  degree  of  conservatism  and  inertia, 
the4  relation  of  the  colors  and  musical  sounds 
was  indefinite,  unsatisfactory,  and  distracting.  Con- 
sidering that  the  experimental  work  has  not  yet 
been  done  which  should  form  a  basis  for  expres- 
sion and  arousing  emotion  by  means  of  colors,  no 
other  outcome  of  superficially  relating  colors  to 
sound  music  could  have  been  expected.  Even 
though  this  be  an  extremely  progressive  age,-  it 
is  not  likely  that  color  music  can  evolve,  in  an 
acceptable  form,  from  the  imagination  of  a  few 
persons. 

77.   While  it  appears  that  the  art  of  mobile  color 
must    evolve    from    fundamental    experimental   data 


THE  ART   OF  MOBILE   COLOR 


317 


TABLE   XXI 
Color  Codes 


Rimington 


Scriabine 


C  Deep  red 

C#  Crimson 

D  Orange-crimson 

D#  Orange 

E  Yellow 

F  Yellow-green 

F#  Green 

G  Bluish  green 

G#  Blue-green 

A  Indigo 

A#  Deep  blue 

B  Violet 

C  Invisible 


Red 

Violet 

Yellow 

Glint  of  steel 

Pearly  blue  and  shimmer  of  moonshine 

Dark  red 

Bright  blue 

Rosy  orange 

Purple 

Green 

Glint  of  steel 

Pearly  blue  and  shimmer  of  moonshine 


on  the  ' emotive  value'  of  colors,  of  simultaneous 
and  successive  contrasts  in  brightness  and  hue,  of 
sequences  in  hues,  tints,  and  shades,  of  rhythm,  etc., 
it  is  interesting  also  to  experiment  with  colors  in  rela- 
tion to  music.  However,  a  safe  elementary  procedure 
in  the  latter  experiments  is  to  use  colored  light  merely 
to  provide  the  *  atmosphere'  and  gradually  to  intro- 
duce the  element  of  varied  intensity  and,  possibly, 
rhythm.  Certainly  it  is  far  less  presumptuous  to  use 
color  in  this  manner  in  the  absence  of  experimental 
data  than  to  attempt  to  play  a  'tune'  in  colors  as  a 
part  of  a  musical  score.  If  it  is  only  a  matter  of 
individual  taste,  any  procedure  is,  perhaps,  legitimate, 
but  when  the  object  is  to  develop  an  art  of  mobile 
color  only  cautious  procedure  is  commendable.  In 
providing  'atmosphere'  for  a  particular  motif  such 
superficial  associational  relations  as  blue-green  for 
rippling  water  and  red  for  fire  (because  artists  paint 


318  COLOR  AND  ITS  APPLICATIONS 

them  thus)  are  insufficient.  It  is  the  deeper  emo- 
tional relation  that  is  desired  which,  perhaps,  cannot 
be  determined  with  certainty  without  many  careful 
experiments  on  a  large  number  of  subjects. 

In  developing  an  independent  art  of  mobile  color, 
what  procedure  shall  be  adopted?  Certainly  the 
fundamental  experiments  will  be  found  to  lie  largely 
in  the  realm  of  psychology.  The  aim  of  the  modern 
artist  is  not  totally  unrelated  to  the  subject,  and  a 
group  of  such  artists  perhaps  would  form  a  most 
interested  audience  for  such  experiments.  The  new 
movement  in  the  theater  which  is  striving  for  har- 
mony in  action,  lighting,  and  setting  is  not  wholly 
unrelated  to  the  subject  under  consideration.  In 
experimenting  with  colors  for  the  purpose  of  devel- 
oping an  art  of  mobile  color  it  may  be  profitable  and 
encouraging  to  study  the  evolution  of  sound  music. 
In  Baltzell's  'History  of  Music'  we  read 

'  When  we  think  of  music  we  have  in  mind  an  organization  of 
musical  sounds  into  something  definite,  something  by  design,  not 
by  chance,  the  product  of  the  working  of  the  human  mind  with 
musical  sounds  and  their  effects  upon  the  human  sensibilities. 
So  long  as  man  accepted  the  various  phenomena  of  musical  sounds 
as  isolated  facts,  there  could  be  no  art.  But  when  he  began  to 
use  them  to  minister  to  his  pleasure  and  to  study  them  and  their 
effects,  he  began  to  form  an  art  of  music.  The  story  of  music  is 
the  record  of  a  series  of  attempts  on  the  part  of  man  to  make 
artistic  use  of  the  material  which  the  ear  accepts  as  capable  of 
affording  pleasure  and  as  useful  in  expressing  the  innermost 
feelings.' 

The  leading  principles  in  music  are  rhythm,  melody, 
harmony,  and  tone  quality,  and  in  the  execution  of  a 
musical  composition  dynamic  contrast  is  an  essential 
factor  in  expression. 

'For  ages  after  the  birth  of  music,  rhythm  and  melody  were 
the  only  real  elements,  rhythm  being  first  recognized.  Music 


THE  ART  OF  MOBILE  COLOR  319 

that  lacks  a  clearly-defined  rhythm  does  not  move  the  masses. 
It  was  not  until  harmony  appeared  that  music  was  able  to  claim  a 
position  equal  to  that  accorded  to  poetry,  painting,  sculpture,  and 
architecture.'  *  These  principles,  rhythm,  melody,  and  harmony, 
became,  when  couched  in  the  forms  of  expression  adopted  by  the 
great  masters,  what  we  call  modern  music,  and  the  story  is  one 
of  a  development  from  extreme  simplicity  to  the  complexity 
illustrated  in  modern  orchestral  scores.' 

The  lesson  we  gain  from  the  foregoing  is  to  proceed 
patiently.  Sound  music  had  an  elementary  begin- 
ning evolving  into  its  present  form  only  after  many 
centuries  of  experiment. 

A  thought  that  naturally  comes  to  us  is  this:  Is 
there  anything  in  Nature  that  suggests  color  music? 
Perhaps  scenes  full  of  color  are  suggestive  of  '  atmos- 
phere' colors  for  musical  compositions.  Perhaps  if 
the  cycle  of  appearances  of  such  a  scene  throughout 
a  day  were  compressed  into  a  period  of  five  minutes, 
it  might  suggest  what  a  composition  in  color  music 
would  be.  Being  unaccustomed  to  thinking  of  color 
apart  from  form,  perhaps  such  studies  would  be 
fruitful.  Certainly  at  first,  in  thinking  of  color  for 
color's  sake  alone,  one  has  a  feeling  that  all  solid 
foundation  has  been  removed  from  beneath  him. 

When  it  comes  to  experimental  work  one  feels 
that  the  foundation  has  been  restored,  but  is  appalled 
at  the  immensity  of  the  work  to  be  done.  The  avail- 
able psychological  literature  yields  some  interesting 
information.  Some  work  on  affection  pertaining  to 
colors  has  been  done,  and  the  studies  of  rhythm  are 
very  extensive;  however,  the  work,  which  eventually 
will  form  a  definite  basis  for  developing  an  art  of 
mobile  color,  has  hardly  been  begun.  The  meager 
data  in  color  preference  partially  described  in  #66 
were  obtained  as  a  beginning  of  an  inquiry  into  some 
of  the  elementary  impressions  produced  by  colors. 


320  COLOR  AND  ITS  APPLICATIONS 

It  appears  from  this  work,  which  supports  conclusions 
arrived  at  by  others,  that  in  general  saturated  colors 
are  more  preferred  than  tints  or  shades,  the  latter 
perhaps  being  generally  more  preferred  than  tints. 
There  is  some  evidence  that  subjects  who  are  less 
capable  of  isolating  the  colors,  that  is,  more  inclined 
to  associate  them  with  other  experiences,  prefer  the 
tints  and  shades  or  so-called  *  artistic'  colors.  Some 
study  has  been  made  of  combinations  of  colors,  but 
without  definite  results  at  the  present  time.  Of 
course  all  the  known  principles  of  harmony  and  con- 
trast of  colors  are  available  for  use  by  the  pioneer 
in  the  art  of  mobile  color.  However,  no  application 
of  these  principles  can  be  made  until  extensive  ex- 
periments have  been  performed.  The  *  emotive  value ' 
of  various  hues,  tints,  and  shades,  of  simultaneous 
and  successive  contrasts  in  hue  and  brightness,  and 
of  rhythmic  sequences  in  hue  and  brightness  must 
be  determined.  Bradford  found  that  saturated  colors 
were  most  preferred  and  that  the  admixture  of  small 
proportions  of  another  color  have  a  lowering  effect 
upon  the  preference  of  a  color.  Regarded  objectively, 
the  pure  colors  were  found  first  in  the  preference 
order  while  those  which  appear  to  be  adulterated 
with  another  color,  were  placed  last.  Cohn  had  pre- 
viously claimed  that  increase  in  saturation  tended 
to  make  a  color  more  pleasing.  Titchener  obtains 
results  of  a  similar  nature  with  the  majority  of  his 
subjects,  who  definitely  reject  tints  and  shades  of 
colors  in  favor  of  the  saturated  colors.  While  a 
color  may  be  most  highly  preferred  among  a  large 
number  of  colors  the  *  emotive  value'  of  this  color 
is  perhaps  rather  low  as  compared  with  many  other 
things.  For  instance  a  dark  blue  color  may  be  dis- 
tinctly more  preferred  than  any  other  color  in  a  cer- 


THE  ART   OF  MOBILE   COLOR  321 

tain  group,  yet  it  can  hardly  be  compared  in  emotive 
value  to  a  song  by  one  of  our  operatic  artists.  As 
Titchener  states,  when  compared  in  pleasantness  with 
a  good  dinner  or  the  scent  of  a  flower  the  color  patch 
will  seem  practically  indifferent.  Of  course  results  of 
impressions  are  only  relative  and  there  is  perhaps 
sufficient  emotive  value  in  colors  alone  to  afford 
pleasure  when  combined  to  form  color  music.  How- 
ever, the  foregoing  point  is  of  interest  in  combining 
colors  and  sound  music.  Certainly  a  *  color  instru- 
ment' cannot  compete  with  a  symphony  orchestra, 
which  leads  to  the  tentative  conclusion  that  color  in 
such  a  relation  should  be  subordinated  to  the  role  of 
merely  providing  '  atmosphere.'  A  *  color  instrument' 
of  definite  form  is  conspicuous  in  its  feebleness  when 
in  the  midst  of  a  symphony  orchestra.  It  was  sug- 
gested that  the  colors  be  used  in  the  rendition  of 
1  Prometheus'  by  combining  them  on  the  whole 
background  of  the  orchestra  setting  without  any  arbi- 
trary limits,  thus  providing  the  atmosphere.  The  use  of 
diaphanous  curtains,  draped  in  loose  folds  and  per- 
haps kept  moving  gently  by  electric  fans  placed  at  a 
considerable  distance,  was  recommended.  However, 
neither  of  these  suggestions  was  adopted,  the  colors 
having  been  played  on  a  relatively  small  white  screen. 
78.  The  mechanical  construction  of  experimental 
apparatus  for  studying  ' color  phrases'  is  simple. 
There  are  two  general  methods  of  procedure  which 
immediately  occur  to  the  experimenter.  In  one,  the 
various  colors  composing  a  *  color  chord'  are  separated 
physically  by  playing  them  on  different  parts  of  a 
white  screen,  thus  introducing  the  factor  of  harmony 
and  overcoming  the  lack  of  analytic  ability  of  the 
visual  apparatus.  In  the  other  the  component  colors 
of  a  color  chord  are  mixed  by  superposition.  Obvi- 


322  COLOR  AND  ITS  APPLICATIONS 

ously,  in  the  latter  case  harmony  is  limited  to  the 
presentation  of  colors  successively  and  the  predomi- 
nant factor  in  'composing'  color  music  to  be  rendered 
by  such  an  instrument  would  be  that  of  color-mixture. 
In  the  former  case  the  predominant  factor  would 
be  that  of  the  harmony  of  colors.  In  both  proce- 
dures the  element  of  rhythm  and  variation  in  bright- 
ness can  be  introduced.  A  decision  regarding  the 
mode  of  presenting  colors --by  juxtaposition  or 
superposition  —  must  be  made  before  any  serious 
attempts  at  composing  color  music  can  be  made. 
Doubtless  instruments  employing  both  principles 
should  be  investigated,  and  with  this  in  mind  two 
simple  instruments  were  constructed.  One  similar 
to  that  illustrated  in  Fig.  127  was  used  by  Rimington, 
who  employed  arc  lamps  for  sources  of  light.  The 
various  colors  indicated  in  Table  XXI  were  played  in 
arbitrarily  selected  positions  relative  to  each  other. 
Obviously  no  purples  appeared  when  the  colors  of  the 
Rimington  scale  were  played  in  this  manner.  This 
omission  is  inexcusable,  for  purple  is  of  a  definite  hue 
and  perhaps  nearly  as  full  of  emotive  value  as  any 
spectral  color.  The  colors  could  also  be  mixed  on  a 
screen.  A  mechanical  dimming  apparatus  was  em- 
ployed for  controlling  the  brightness  of  the  colors. 
Rimington  evidently  has  experimented  considerably 
with  such  an  apparatus,  but  gives  little  data  that 
supplies  fundamental  information  from  which  to 
develop  an  art  of  mobile  color.  Such  an  instrument 
was  constructed  by  the  author,  using  tungsten  incan- 
descent lamps  and  fairly  pure  color  filters,  the  wiring 
diagram  being  as  shown  in  Fig.  127.  Either  mechani- 
cal or  electrical  dimmers  may  be  used  for  control- 
ling the  brightness  of  the  colors.  A  similar  instru- 
ment was  used  in  the  rendition  of  the  'Luce'  score 


THE  ART   OF  MOBILE   COLOR 


323 


in  *  Prometheus,'  cited  early  in  the  present  chapter. 
In  order  to  overcome  the  arbitrariness  of  the  relative 
positions  at  which  the  colors  appeared  upon  the  white 
reflecting  screen  an  oscillating  motion  was  given  to 
the  colors.  By  this  means  the  colors  never  appeared 
completely  superposed  and  appeared  on  various  occa- 
sions on  different  parts  of  the  screen. 


•* 

c? 

1 

^ 

1 

Qj       ^ 
^       ^ 

1 

^ 

Crimso 

\ 

I 

Yellow 

I   5 

>      ^ 

Oii            % 

x    vS 

j  |    f 

X    ^ 
1  -^ 
Q     ^ 

. 

\ 

\           \ 

i 

\      \ 

J 

i 

i 

\ 

J    uJ     * 

J 

m  $m 

V 

1  —  ,T~ 

*9 

i...   .  .•; 

m 

t  1.._  ,._•-; 

t    —  1  ^-=1"     —4 

Fig.   127.  —  Illustrating  an  instrument  for  studying  the  emotive  or  affective 
value  of  colors  and  color  phrases ;  Rimington's  color  code  is  also  shown. 

Another  instrument  constructed  by  the  author,  on 
the  principle  that  any  color  can  be  matched  by  a 
mixture  of  three  primary  colors,  namely  red,  green, 
and  blue,  is  illustrated  in  Figs.  128  and  129.  Red, 
green,  blue,  and  clear  tungsten  lamps  are  respectively 
placed  in  series  with  specially  constructed  resistors. 
Each  of  the  resistors,  a,  fe,  c,  and  d,  contain  ten  mov- 
able contacts  which  are  respectively  connected  to  the 
corresponding  keys  on  the  keyboard.  On  pressing 
a  given  key  the  circuit  is  completed  through  the 


324 


COLOR  AND   ITS  APPLICATIONS 


corresponding  lamps  and  a  certain  amount  of  resist- 
ance wire.  The  line  voltage  is  applied  at  7,  and  C 
is  a  common  terminal  for  the  four  circuits.  The 
clear  tungsten  lamps  are  in  reality  *  daylight'  lamps, 
thus  producing  white  light.  This  light  is  used  to 
dilute  the  colored  light  to  any  degree  of  saturation 
represented  by  the  ten  steps  in  intensity  produced 


Fig.  128.  —  A  color-mixture  instrument  for  studying  the  emotive  and  affective 
value  of  colors  and  color  phrases. 

by  pressing  the  corresponding  keys  in  the  upper  row 
marked  W.  Thus  ten  steps  in  intensity  can  be 
obtained  for  light  of  each  primary  color  and  white. 
Such  a  combination  is,  of  course,  arbitrary,  but  is 
sufficiently  elaborate  for  preliminary  experimental 
purposes.  Hundreds  of  different  colors  are  obtain- 
able, varying  in  brightness  from  that  just  perceptible 
to  the  maximum  brightness,  which  is  at  the  limit  of 
comfortableness.  The  lamps  are  placed  inside  a 


THE  ART   OF  MOBILE   COLOR 


325 


velvet-lined  box  (Fig.  129)  around  the  rectangular 
aperture.  The  colors  are  mixed  by  superposition 
and  viewed  at  present  on  a  circular  white  diffusing 
surface  placed  on  the  back  of  the  box  opposite  the 
viewing  aperture.  The  movable  contacts  are  adjusted 
so  that  any  corresponding  set  of  three  keys  in  the 
/?,  G,  and  B  rows  will  produce  white  light.  Thus 
white  light  of  ten  degrees  of  brightness  can  be  made 


B      G      R      G      B 

O    O    O    O    O 


o 


B      R      G      R      B 


._.X- 


PROMT  VIEW 


END-VIEW 


Fig.  129.  —  Showing  the  relative  positions  of  the  colored  lamps  in  the  apparatus 
diagrammatically  shown  in  Fig.  128 

in  this  manner.  The  upper  row  of  keys  for  producing 
white  light  has  been  installed  in  order  to  produce 
greater  flexibility. 

Considerable  personal  experimenting  has  been 
done  with  these  forms  of  apparatus,  but  little  definite 
information  has  yet  been  derived.  The  foregoing  has 
been  presented  to  illustrate  the  procedure  considered 
desirable  in  this  work.  The  amount  of  experiment- 
ing that  can  be  done  with  such  apparatus  is  very  ex- 
tensive, but  the  first  question  to  decide  concerns  the 
character  of  the  data  desired.  Many  have  dreamed 


326  COLOR  AND   ITS  APPLICATIONS 

of  color  music,  some  have  written  about  it,  and  a  few 
have  attempted  to  present  it.  The  objects  of  this 
discussion  have  been  to  show  that  there  is  no  art  of 
mobile  color  at  present;  that  meager  constructive 
data  exists  concerning  it;  that  there  have  been 
hardly  more  than  superficial  attempts  made  to  present 
it;  that  psychological  studies  must  be  relied  upon  to 
point  the  way  toward  its  development;  that  it  is  a 
field  worthy  of  cultivation;  and  that  there  are  defi- 
nite problems  that  must  be  studied  in  order  to  obtain 
foundation  material  for  building  up  an  art  of  mobile 
color. 

REFERENCES 

E.  J.  G.  Bradford,  On  the  Relation  and  Aesthetic  Value  of  the 
Perceptive  Types  in  Color  Appreciation,  Amer.  Jour,  of  Psych.  1913, 
24,  p.  545. 

J.  Cohn,  Gefuhlston  und  Sattigung  der  Farben,  Phil.  Stud.  1900, 
15,  p.  279. 

D.  R.  Major,  On  the  Affective  Tone  of  Single  Sense  Impression, 
Amer.  Jour.  Psych.  1895,  6,  p.  57. 

W.  H.  Winch,  Color  Preferences  of  School  Children,  Brit.  Jour. 
Psych.  1909,  3,  p.  42. 

L.  R.  Geissler,  Experiments  in  Color  Saturation,  Amer.  Jour,  of 
Psych.  1913,  24,  p.  171. 

E.  B.  Titchener,  Experimental  Psychology,  New  York,  1910,  p.  149. 
A.  W.  Rimington,  Colour-Music,  London. 

C.  A.  Ruchmich,  A  Bibliography  of  Rhythm,  Amer.  Jour,  of  Psych. 
1913,  24,  p.  508. 

G.  H.  Clutsam,  The  Harmonies  of  Scriabine,  London  Musical 
Times,  March,  1913,  p.  157. 

For  a  discussion  of  the  rendition  of  'Prometheus'  with  an 
accompaniment  of  colors,  see  New  York  papers  of  March  22,  1915. 

J.  D.  MacDonald,  Sounds  and  Colours,  1867. 

J.  Aitken,  On  Harmony  of  Colour,  Trans.  Roy.  Scot.  Soc.  of 
Arts  IX,  1873. 

Mrs.  E.  J.  Hughes,  Harmonies  of  Tones  and  Colours,  1883. 

Arnold  Ebet,  Farbensymphonie,  Alleg.  Musik  Zeit.  1912,  39, 
Nos.  34  and  35. 

M.  Luckiesh,  The  Language  of  Color,  1918. 


CHAPTER   XVI 
COLORED   MEDIA 

79.  Available  Coloring  Materials. — In  any  kind 
of  work  a  knowledge  of  the  tools  and  materials  avail- 
able is  quite  important.  If  one  may  judge  from  the 
questions  that  are  asked  by  many  interested  in  va- 
rious phases  of  color  science,  a  brief  outline  of  colored 
media  and  means  of  manipulating  them  should  be  of 
interest.  The  available  coloring  materials  are  very 
numerous,  yet  it  is  often  difficult  to  find  satisfactory 
pigments  for  a  given  purpose.  It  is  of  considerable 
advantage  to  have  at  hand  a  large  variety  of  these 
materials;  therefore  a  list  of  useful  colored  media  are 
presented  below. 

Colored  glasses. --Sets  of  samples  can  be  ob- 
tained from  various  supply  houses.  Signal  glasses 
afford  a  limited  number  of  fairly  pure  colors,  usually 
red,  yellow,  green,  blue-green,  blue,  and  purple. 

Colored  gelatines.  -  -  Very  elaborate  sets  of  colored 
gelatines  can  be  obtained  from  theatrical  supply 
houses.  These  are  exceedingly  useful,  though  lack- 
ing in  permanency.  If  mounted  between  sheets  of 
glass  and  kept  in  a  ventilated  position,  many  of  them 
will  be  fairly  durable.  Complete  sets  of  samples  are 
very  convenient. 

Colored  lacquers.  —  Those  intended  for  coloring 
incandescent  lamps  are  very  useful,  although  it  is 
often  desirable  to  mix  these  carefully  in  order  to 
obtain  colors  of  greater  spectral  purity.  Such  col- 
ored lacquers  vary  considerably  in  permanency,  and 

327 


328  COLOR  AND  ITS  APPLICATIONS 

wherever  possible  it  is  well  to  apply  the  coloring  to 
sheets  of  glass  which  can  be  mounted  at  some  dis- 
tance from  the  lamp.  This  insures  a  much  greater 
permanency. 

Aniline  Dyes.  —  For  coarse  work  the  cheap  dyes 
used  for  coloring  cloth  will  afford  a  fairly  satisfactory 
range  of  hues.  By  judiciously  mixing  these  dyes 
some  fairly  pure  colors  can  often  be  obtained,  al- 
though as  mixing  usually  tends  to  produce  muddy 
colors  the  better  procedure  is  to  have  at  hand  a 
variety  of  fundamental  pigments  from  which  perhaps 
a  satisfactory  color  can  be  selected.  A  variety  of 
dyes  of  the  better  grade  is  almost  indispensable  for 
accurate  color  work.  Such  dyes  are  usually  pure 
and  fairly  reproducible,  and  are  the  best  coloring 
media  for  making  photographic  and  other  screens 
requiring  pure  colors.  Sets  of  stains  for  tinting 
lantern  slides  are  available.  These  coloring  media 
can  be  purchased  in  various  forms,  liquid,  powder, 
sheets,  etc.  It  is  probably  surprising  to  the  unini- 
tiated what  a  variety  of  coloring  materials  can  be 
obtained  in  the  stores  of  a  city  of  moderate  size. 

Artists'  Pigments.  —  Such  pigments  are  classed 
as  pastel,  water  colors,  and  oil  paints.  These  all 
have  their  uses  in  color  science.  Water  colors  are 
now  available  in  opaque  moist  pastes,  which  have 
advantages  in  some  classes  of  work. 

Printers'  Inks.  —  Such  a  set  of  pigments  will  be 
found  useful  by  those  desiring  to  collect  a  variety  of 
coloring  media.  They  are  especially  adaptable  to 
applications  similar  to  those  found  in  the  print  shop. 

Colored  Papers.  —  The  ordinary  colored  tissue 
papers  are  useful  in  demonstrating  color  effects,  but 
in  the  study  of  the  science  of  color  no  series  is  equal 
in  purity  and  uniformity  to  the  imported  colored 


COLORED  MEDIA  329 


papers,  such  as  the  Wundt  colored  papers  supplied 
by  Zimmerman  which  are  mentioned  in  this  work 
on  various  occasions. 

80.  Pigments.  —  As  stated  in  #  72  pigments  are 
derived  from  mineral,  animal,  and  vegetable  matter, 
and  in  general  the  inorganic  pigments  are  the  most 
durable.  The  organic  dyes  are  often  more  brilliant, 
and  for  a  great  many  purposes  are  more  satisfactory, 
than  inorganic  pigments,  because  the  latter  are  usu- 
ally more  opaque.  The  durability  of  pigments  is  a 
matter  of  degree,  and  depends  upon  the  protection 
provided  against  moisture  and  other  destructive 
agents  in  the  atmosphere,  such  as  gases  and  smoke* 
Few  pigments  will  withstand  excessive  amounts  of 
heat  and  light.  An  extended  discussion  of  pigments 
is  outside  the  scope  of  this  chapter,  but  a  few  details 
regarding  common  pigments  should  prove  helpful. 
The  chemistry  of  pigments  obviously  is  complex,  so 
no  simple  rules  can  be  formulated  which  will  always 
guide  the  colorist  in  making  mixtures  of  pigments 
that  will  not  interact. 

Blue.  —  In  general  blue  pigments  reflect  or  trans- 
mit an  appreciable  amount  of  deep  red  rays,  which 
becomes  quite  noticeable  under  ordinary  artificial 
light.  Ultramarine  is  considered  by  the  artist  to  be 
a  close  approach  to  spectral  blue  in  hue,  yet  it  trans- 
mits a  considerable  proportion  of  red  rays  (Fig.  103, 
122).  Natural  ultramarine  is  obtained  from  a  min- 
eral, but  owing  to  its  scarcity  an  imitation  has  been 
produced  artificially  in  various  grades.  The  artificial 
ultramarine  is  quite  permanent  and  is  insoluble  in 
water,  alcohol,  turpentine,  and  oil.  Ultramarine  ash 
is  a  blue-gray  pigment  derived  as  a  by-product  in 
the  preparation  of  natural  ultramarine. 

Cobalt-blue  is  readily  prepared  quite  pure  and  is 


330  COLOR  AND  ITS  APPLICATIONS 

very  durable,  although  it  is  far  from  a  pure  blue. 
Its  reddish  appearance  under  artificial  light  indicates 
that  it  reflects  a  large  proportion  of  deep  red  rays, 
which  conclusion  is  supported  on  analyzing  the  re- 
flected light  (see  6,  Fig,  122).  Smalte  is  a  powdered 
cobalt  glass  of  a  brilliant,  transparent  color  that  is 
quite  durable. 

Prussian  blue,  like  most  artificial  pigments,  varies 
in  quality.  It  is  not  generally  as  durable  as  the  pre- 
ceding blue  pigments,  but  is  fairly  permanent  if  used 
alone.  It  interacts  with  many  pigments.  In  oxalic 
acid  it  forms  a  satisfactory  writing  fluid.  It  can  be 
made  by  mixing  a  ferric  salt  with  potassium  ferro- 
cyanide.  It  can  be  deposited  intimately  in  contact 
with  a  fabric  if  the  latter  be  dipped  first  into  one 
solution  and  then  into  the  other.  An  excess  of  the 
potassium  ferrocyanide  forms  a  compound  known  as 
soluble  prussian  blue. 

Indigo  is  derived  from  the  vegetable  kingdom 
and  belongs  to  the  lakes.  It  is  insoluble  in  water, 
ether,  oils,  and  cold  alcohol.  It  dissolves  in  boiling 
concentrated  alcohol  and  fuming  sulphuric  acid.  In 
the  latter  solvent  it  forms  saxon  blue. 

There  are  numerous  blue  aniline  dyes,  but  most 
of  them  transmit  red  rays  as  well  as  blue. 

Green.  --  Chromium  oxide  is  a  durable,  opaque, 
deep  green  pigment. 

Emerald-green  usually  is  a  carbonate  of  copper 
mixed  with  alumina.  It  is  quite  opaque  and  durable 
and  of  a  brilliant  green  color. 

Many  greens  are  made  by  mixtures  of  such  pig- 
ments as  chrome-yellow  and  prussian  blue,  but  the 
luminosity  of  such  a  mixture  depends  upon  the 
amounts  of  green  in  the  two  pigments  (#72). 

Terra   verte,   a   native   mineral   found   in   various 


COLORED  MEDIA  331 


parts  of  Europe,  is  opaque  and  durable  and  quite 
satisfactory  in  color. 

Malachite  green  is  a  natural  carbonate  of  copper. 
It  is  pale  green  in  color  and  moderately  permanent. 
This  pigment  is  being  imitated  artificially. 

There  are  several  beautiful  greens  among  the 
organic  dyes,  but  of  less  durability. 

Yellow.  --  Gamboge  closely  represents  spectral 
yellow.  It  is  a  gum  resin  employed  very  extensively 
in  water  colors.  It  is  a  bright,  transparent,  per- 
manent yellow. 

Cadmium-yellow  is  a  brilliant,  opaque  pigment 
which  forms  fairly  satisfactory  greens  by  mixing 
with  a  number  of  the  greenish  blue  pigments.  It 
contains  sulphur,  and  therefore  should  not  be  mixed 
with  pigments  containing  lead.  It  is  satisfactory  in 
combination  with  zinc-white. 

Indian  yellow  is  a  permanent,  fairly  transparent, 
orange-yellow.  The  pure  pigment  burns  easily,  which 
is  a  means  of  detecting  fraudulent  adulteration  or 
substitution. 

Chrome-yellow  is  lead  chromate  and  varies  in 
color  from  a  lemon-yellow  to  a  deep  orange,  depend- 
ing upon  the  chemical  constitution  and  the  admixture 
of  other  substances.  It  is  used  in  both  oil  and  water 
colors. 

Zinc  chromate  is  an  opaque,  permanent  yellow 
pigment  which  mixes  well  with  other  pigments. 

Potassium  bichromate  is  a  permanent  yellow 
having  many  uses.  It  dissolves  in  water  and  appears 
a  greenish  yellow  in  slight  concentration,  but  ap- 
proaches a  deep  amber  in  a  saturated  solution. 

Satisfactory  spectral  yellows  are  rare,  even  among 
the  large  number  of  organic  dyes  available.  Tart- 
razine,  aurantia,  martius-yellow,  and  naphthol-yellow 


332  COLOR  AND  ITS  APPLICATIONS 

are  representative  of  these  dyes.  They  have  a  green- 
ish tinge. 

The  ochres,  which  are  earthy  combinations  of  iron 
oxides,  yield  several  yellow  pigments.  The  native 
ochres  are  yellow  and  red. 

Red. —  Carmine,  which  is  obtained  from  the  cochi- 
neal insect,  closely  imitates  spectral  red,  and  is  con- 
sidered by  many  as  the  most  beautiful  red  pigment 
known.  It  is  opaque  and  mixes  well  with  other  pig- 
ments but  is  not  very  permanent. 

Vermilion  is  a  natural  compound  of  sulphur  and 
mercury  found  in  many  places,  and  in  mineralogy  is 
called  cinnabar.  It  is  available  in  several  hues,  vary- 
ing from  orange-red  to  deep  red  in  both  oil  and 
water  colors. 

Indian  red  and  Venetian  red  are  native  ochres. 
Some  of  the  yellow  ochres  are  converted  into  light 
red  pigments  by  calcining. 

The  madder  pigments,  which  are  lakes,  include 
various  reds.  The  coloring  matter  is  extracted  from 
roots  and  united  with  alumina.  These  pigments 
are  not  very  permanent. 

Lakes.  —  The  coloring  elements  used  in  lakes  are 
generally  of  vegetable  origin.  These  possess  the 
property  of  being  precipitated  from  .an  aqueous  solu- 
tion by  metallic  oxides,  with  which  they  combine. 
They  have  alumina,  and  sometimes  other  oxides 
associated  with  them,  for  a  base  to  give  them  body. 
If  it  were  not  for  the  affinity  of  these  oxides  for  many 
organic  coloring  matters  many  colors  would  not  be 
available.  For  example,  indian  lake  contains  a  col- 
oring matter  extracted  from  lac;  the  coloring  element 
in  yellow  lake  is  derived  from  berries;  and  the  color- 
ing matters  in  cochineal  and  madder  are  extracted 
as  described  above. 


COLORED  MEDIA  333 


White.  —  White  pigments  are  used  in  diluting 
colored  pigments  for  obtaining  tints. 

White  lead  is  carbonate  of  lead.  It  has  many 
commercial  names,  but  perhaps  flake  white  is  the 
most  common.  It  is  a  very  opaque  pigment.  Oil 
gives  it  a  yellowish  tint  and  should  not  be  used  very 
freely  when  a  pure  white  surface  is  desired.  White 
lead  is  attacked  by  sulphur  and  converted  into  black 
lead  sulphide.  It  is  more  liable  to  react  with  other 
pigments  than  zinc-white,  which  is  a  formidable  rival. 

Zinc-white  is  oxide  of  zinc.  It  possesses  all  of 
the  good  qualities  of  white  lead  and  perhaps  none 
of  the  objectionable  features.  It  is  claimed  that  the 
covering  power  of  zinc-white  is  greater  than  that  of 
white  lead.  Its  sulphide  is  white,  so  that  sulphur  does 
not  discolor  it. 

Black.  —  Black  pigments  are  usually  carbons. 
Ivory-black  is  obtained  from  ivory  waste  and  pos- 
sesses a  rich  black  appearance.  It  produces  excel- 
lent grays  when  mixed  with  white.  Bone-black  is  a 
cheaper  substitute. 

Lamp-black  is  obtained  by  burning  certain  sub- 
stances in  an  atmosphere  containing  little  air  or  oxygen. 
Kerosene  and  coal  gas  yield  soot  which  makes  a  satis- 
factory black. 

Nigrosine  is  a  black  pigment,  soluble  in  water, 
which  is  very  useful.  It  can  be  readily  incorporated 
into  various  mediums  and  makes  a  fairly  satisfactory 
neutral  tint  screen  in  a  gelatine  film. 

81.  Solvents. — In  making  lacquers  various  sol- 
vents are  available,  the  properties  of  some  of  them 
being  given  below.  (See  Table  III.) 

Methyl  alcohol  (wood  alcohol)  mixes  with  water 
in  all  proportions.  It  is  similar  to  grain  alcohol  as 
a  solvent. 


334  COLOR  AND  ITS  APPLICATIONS 

Ethyl  alcohol  (grain  alcohol)  dissolves  many 
resins,  oils,  soaps,  glycerol,  camphor,  celluloid, 
phenol,  iodine,  and  many  chlorides,  iodides,  bro- 
mides, and  acetates. 

Acetone  dissolves  fats,  oils,  gums,  resins,  celluloid, 
and  camphor,  and  mixes  in  ethyl  alcohol  and  water. 

Ether,  produced  by  distilling  alcohol  and  sulphuric 
acid  in  proper  proportions,  dissolves  fats,  oils,  resins, 
iodine,  bromine,  and  many  alkaloids.  It  mixes  with 
alcohol,  benzine,  chloroform,  and  slightly  with  water. 

Amyl  alcohol  mixes  with  benzol,  ether,  alcohol, 
and  slightly  with  water.  It  dissolves  oils,  camphor, 
resins,  alkaloids,  and  iodine. 

Amyl  acetate  (artificial  banana  oil)  mixes  in  all 
proportions  with  alcohol,  amyl  alcohol,  and  ether. 
It  dissolves  celluloid  and  is  used  in  the  preparation 
of  collodion  varnishes. 

Benzine  should  not  be  confused  with  benzene  or 
benzol,  the  latter  being  derived  from  coal  tar.  It  is 
a  substitute  for  turpentine  in  paints,  pils,  and  driers. 

Glacial  acetic  acid  (pure  acetic  acid)  mixes  with 
water,  alcohol,  and  ether.  It  dissolves  oils,  phenols, 
resins,  and  gelatine. 

Linseed  oil  is  used  as  a  vehicle  in  oil  pigments. 
It  dissolves  hard  resins,  amber,  and  copal  and  is  used 
for  making  varnishes. 

Poppy  oil  replaces  linseed  oil  in  oil  pigments  where 
the  yellow  color  of  the  linseed  oil  is  objectionable. 

Benzene  is  derived  from  coal  tar.  It  mixes  with 
alcohol,  ether,  petrolic  ether,  turpentine,  and  dissolves 
oils,  fats,  waxes,  iodine,  and  rubber.  It  loosens  paint. 
Benzol  is  an  impure  benzene.  Toluol,  toluene,  and 
methyl-benzol  are  similar  to  it. 

Gelatine  is  soluble  in  hot  water  and  concentrated 
acetic  acid,  forming,  in  the  latter  case,  an  adhesive 


COLORED  MEDIA  335 


paste.  Potassium  bichromate  renders  it  insoluble 
on  exposure  to  light.  Formalin  added  to  a  warm 
aqueous  solution  and  permitted  to  dry  renders  the 
gelatine  insoluble  in  hot  water. 

Turpentine  dissolves  fats,  oils,  and  resins.  It 
is  used  for  thinning  paints  and  varnishes. 

Venice  turpentine  is  slowly  soluble  in  absolute 
alcohol,  but  is  readily  soluble  in  ether,  acetone,  pe- 
trolic ether,  benzol,  and  glacial  acetic  acid.  It  is 
used  in  fixing  colors,  in  printing  inks,  and  in  spirit 
varnishes  to  give  elasticity. 

Canada  balsam  is  soluble  in  ether,  chloroform, 
petrolic  ether,  benzol,  turpentine,  and  gasoline.  It 
is  used  to  cement  glasses,  and  owing  to  the  fact  that 
its  refractive  index  is  close  to  that  of  glass  it  practi- 
cally eliminates  reflection  and  refraction  of  light  at 
the  surfaces  in  contact  with  it.  For  this  reason  it 
is  excellent  for  cementing  cover  glasses  on  color 
filters. 

82.  Varnishes.  —  A  varnish  is  usually  made  by 
dissolving  a  resin  in  a  medium  such  as  alcohol,  tur- 
pentine, or  oil,  the  first  forming  a  spirit  varnish,  the 
second  a  turpentine  varnish,  and  the  third  an  oil 
varnish.  The  so-called  resins  most  commonly  em- 
ployed are  copal,  sandarac,  mastic,  dammar,  shellac, 
and  amber.  The  properties  of  a  varnish  depend 
largely  upon  the  resin  and  somewhat  upon  the  solvent. 
If  the  solvent  is  volatile,  like  alcohol  and  turpentine, 
after  the  varnish  dries  the  resin  is  left  in  the  same 
state  as  before  it  was  dissolved.  These  are  quick 
drying  varnishes.  If  the  solvent  be  an  oil,  then  both 
the  oil  and  resin  remain  and  the  coating  after  drying 
is  pliable  and  tough. 

Copal  is  soluble  in  hot  linseed  oil;  sandarac  in 
alcohol;  mastic  in  ether,  in  hot  alcohol,  and  in  tur- 


336  COLOR  AND   ITS  APPLICATIONS 

pentine;  dammar  in  alcohol  and  in  turpentine;  shel- 
lac in  alcohol  and  in  a  solution  of  borax;  amber  in 
boiling  linseed  oil;  gum  arabic  in  water  forming  a 
varnish  for  water  colors;  gum  kauri  in  hot  ether,  in 
turpentine,  in  amyl  alcohol,  and  in  benzol;  common 
resin  in  ether,  alcohol,  turpentine,  benzol,  acetone, 
or  hot  linseed  oil. 

Common  resin  in  wood  alcohol  forms  a  cheap 
varnish.  An  excellent  spirit  varnish  is  obtained  by 
dissolving  dammar  in  alcohol  and  turpentine,  the 
proportions  of  the  latter  being  respectively  about  four 
to  one.  A  weather-proof  varnish  can  be  made  of 
dried  copal  7%,  alcohol  15%,  ether  77%,  and  tur- 
pentine 1  %. 

83.  Lacquers.  --  Shellac  dissolved  in  alcohol  and 
decanted  after  settling  provides  a  cheap  lacquer  and 
solvent  for  some  aniline  dyes.  Ordinary  shellac  is 
quite  yellowish  in  color,  so  that  the  use  of  bleached 
shellac  is  sometimes  advisable.  The  latter,  however, 
does  not  dissolve  as  readily  as  the  yellow  shellac, 
but  satisfactory  proportions  are  one  part  of  bleached 
shellac  to  eight  parts  of  90%  alcohol.  In  making 
colored  lacquers  the  aniline  dyes  are  usually  more 
satisfactory  on  account  of  their  transparency,  although 
they  lack  permanency  when  exposed  to  radiant  en- 
ergy. The  inorganic  pigments  are  more  opaque,  but 
are  usually  more  permanent.  In  general  they  do  not 
dissolve,  although  they  can  be  held  in  suspension. 
They  are  not  as  generally  satisfactory  as  the  aniline 
dyes  for  coloring  media,  excepting  for  their  greater 
permanency. 

Photographers'  ordinary  collodion,  which  consists 
•  of    pyroxylin  (soluble    guncotton)   dissolved    in    ether 
and  alcohol,  can  be  used  as  a  solvent  for  aniline  dyes 
for  coloring  incandescent  lamp  bulbs. 


COLORED  MEDIA  337 


Ordinary  photographic  film  (from  which  the  emul- 
sion has  been  removed)  dissolved  in  amyl  acetate, 
alcohol,  or  acetone,  provides  a  satisfactory  lacquer 
for  lamp  colorings.  Ordinary  clear  celluloid  scraps 
containing  a  large  percentage  of  camphor  are  readily 
dissolved  in  acetone,  thus  providing  a  cheap  lacquer 
for  dyeing  purposes.  The  latter  celluloid  scraps  dis- 
solve in  wood  alcohol  and  in  amyl  acetate,  but  when 
the  lacquer  drys  it  becomes  white;  however,  this 
provides  a  means  of  making  a  cheap  though  not  very 
satisfactory  opal  lacquer.  Ordinary  white  celluloid 
scraps  dissolved  in  wood  alcohol  provide  a  very  cheap 
opal  lacquer.  A  permanent  opal  solution  can  be  made 
by  mixing  pure  zinc-white  to  a  fair  consistency,  using 
but  little  oil  with  a  few  drops  of  gold  size.  This  can 
be  applied  by  stippling  with  a  flat-headed  brush. 
Obviously  this  solution  can  be  readily  colored  by 
mineral  pigments,  but  such  mixtures  are  not  very 
transparent. 

If  it  is  desirable  to  make  a  transparent  glass  dif- 
fusing or  translucent,  a  saturated  solution  of  epsom 
salts  in  warm  water  is  satisfactory.  After  applying 
this  solution  and  permitting  it  to  dry,  a  surface  is 
obtained  similar  to  that  produced  by  etching  or  sand 
blasting.  This  can  be  colored  with  some  of  the  dyes 
soluble  in  water.  Such  a  surface  is  not  permanent. 

84.  Dyeing  Gelatine  Films.  —  Perhaps  the  most 
convenient  manner  of  making  color  filters  for  a  large 
variety  of  uses  is  in  applying  the  coloring  matter  to 
gelatine.  A  simple  scheme  is  found  in  placing  a 
photographic  plate  in  an  ordinary  fixing  solution  for  a 
few  moments,  and,  after  thoroughly  washing  it,  per- 
mitting it  to  soak  in  an  aqueous  solution  of  the  dye. 
The  gelatine  coating  will  absorb  considerable  of  the 
dye,  the  depth  of  coloring  being  controlled  chiefly  by 


338  COLOR  AND   ITS  APPLICATIONS 

the  concentration  of  the  colored  solution  and  some- 
what by  the  period  of  time  the  plate  is  permitted  to 
remain  in  the  bath.  If  the  coloring  is  too  dense,  some 
of  it  can  be  washed  out  by  placing  the  plate  in  run- 
ning cold  water.  It  is  sometimes  necessary  to  acidu- 
late the  solution  slightly  or  to  add  ammonia,  alcohol, 
etc.,  in  order  completely  to  dissolve  the  dyes,  but 
this  does  not  usually  interfere  with  the  above  process. 
Better  control  is  obtained  by  adding  an  aqueous 
solution  of  the  dye  to  a  solution  of  gelatine  in  warm 
water  and  flowing  the  dyed  gelatine  on  a  level  plate 
of  glass  or  other  transparent  media.  This  procedure 
lends  itself  to  accurate  reproduction.  It  is  advisable 
to  use  a  harder  variety  of  gelatine,  which  can  be  pur- 
chased from  chemical  supply  houses.  From  four  to 
six  per  cent  of  gelatine  (by  weight)  in  water  is  found 
satisfactory.  The  gelatine  is  permitted  to  soak  in 
cold  water  for  an  hour  or  more ;  then  the  vessel  con- 
taining it  is  placed  in  a  basin  of  water  and  gently 
heated.  It  is  advisable  not  to  heat  the  water  above 
50  deg.  C  or  more  than  is  necessary  to  liquefy  the 
gelatine.  This  solution  should  be  filtered  through 
a  coarse  cloth  free  from  lint,  and  the  plate  should 
be  flowed  in  a  dust-free  atmosphere.  Sometimes 
it  is  well  to  warm  the  glass  plate  before  flowing  the 
gelatine.  The  amount  of  gelatine  solution  should  be 
approximately  one  cubic  centimeter  to  ten  square 
centimeters  of  area.  It  is  well  to  permit  the  plate 
to  dry  uniformly  until  completely  hardened.  The 
surface  will  not  be  optically  plane,  but  where  this  is 
necessary  another  plate  glass  may  be  cemented  on 
top  of  it  with  Canada  balsam  and  a  moderate  pressure 
should  be  applied  for  several  days.  When  dried  at 
a  temperature  of  40  deg.  C,  only  a  day  or  two  is  re- 
quired for  the  balsam  to  harden.  After  the  plates 


COLORED  MEDIA  339 


are  thoroughly  dry  they  can  be  bound  together  at 
the  edges  with  metal  strips  or  gummed  paper.  A  dry- 
ing cabinet  heated  by  means  of  carbon  incandescent 
lamps  is  very  safe  and  convenient,  and  the  tempera- 
ture can  be  readily  regulated  by  varying  the  number 
of  lamps  in  operation. 

Gelatine  sheets  can  be  made  by  flowing  the  gela- 
tine solution  upon  a  level  aluminum  plate,  from  which 
they  are  readily  removed  after  drying.  Doubtless 
there  are  better  processes  for  the,  latter  procedure 
used  in  the  manufacture  of  such  sheets. 

85.  Celluloid. — This  material  is  of  interest  be-  t- 
cause  of  its  use  in  lacquers  and  its  transparency  and 
durability,  which  make  it  a  substitute  for  glass  or 
gelatine  films.  An  undesirable  characteristic,  how- 
ever, is  its  inflammability,  although  tests  indicate 
that  the  commercial  celluloid  is  not  dangerously 
explosive.  It  resists  most  acids  and  bases  of 
moderate  concentration  when  cold.  Glacial  acetic 
acid  rapidly  dissolves  it,  and  when  this  solution  is 
poured  into  water  the  nitrocellulose,  camphor,  and 
other  substances  are  precipitated.  It  dissolves  in 
alcohol,  the  best  solvent  being  camphorated  alcohol 
(10  parts  camphor  to  100  parts  alcohol).  Acetone, 
either  the  liquid  or  vapor,  dissolves  it.  Celluloid 
films  can  be  made  by  casting  or  by  a  continuous 
process,  and  can  be  polished  by  felt  disks  or  rollers, 
using  powdered  pumice  stone,  soap,  or  polishing 
oil. 

Celluloid  takes  up  dye  very  well  from  a  solution 
of  the  coloring  in  alcohol.  The  colors  for  staining 
should  act  like  mordants,  or  their  application  should 
be  similar;  that  is  they  should  penetrate  deeply  into 
celluloid,  thus  coloring  the  mass.  The  usual  solvents 
are  alcohol,  acetone,  acetic  acid,  and  amyl  acetate. 


340  COLOR  AND   ITS  APPLICATIONS 

In  staining  celluloid  it  is  first  moistened  by  a  soften- 
ing agent  in  which  the  aniline  dyes  are  mixed;  then 
on  dipping  the  celluloid  into  such  a  solution  the  dye 
penetrates  the  mass. 

Celluloid  is  readily  colored  by  the  foregoing 
methods,  but  can  also  be  colored  by  means  of  mineral 
dyes,  though  if  transparency  is  desired,  which  is  the 
condition  considered  most  important  here,  these 
colorings  are  not  as  satisfactory,  although  they  pro- 
vide permanent  colors.  A  solution  of  indigo  in  sul- 
phuric acid  and  neutralized  by  potassium  hydroxide 
produces  a  blue  dye.  Another  method  which  fur- 
nishes a  more  satisfactory  blue  results  in  the  pro- 
duction of  Prussian  blue.  The  celluloid  is  immersed 
in  a  bath  of  ferric  chloride,  and  after  drying  is  dipped 
into  a  bath  of  potassium  ferrocyanide.  To  color 
celluloid  green,  it  is  dipped  into  a  solution  of  verdigris 
and  ammonium  chloride.  To  color  it  yellow  it  is 
immersed  in  a  solution  of  lead  nitrate  and  then  dipped 
into  a  solution  of  neutral  potassium  chromate.  Solu- 
tions of  chrysoidine,  auramine,  and  many  aniline  dyes 
in  alcohol  are  satisfactory.  To  color  celluloid  red  it 
may  be  dipped  first  in  a  dilute  solution  of  nitric  acid, 
then  immersed  in  an  ammoniacal  solution  of  carmine. 
Color  will  be  readily  absorbed  by  celluloid  if  its  sur- 
face is  first  sandblasted. 

86.  Phosphorescent  materials.  —  A  variety  of 
phosphorescent  materials  are  available  from  chemical 
supply  houses,  in  varying  degrees  of  purity  and  of 
various  colors.  These  have  their  place  in  colored 
effects,  especially  for  demonstration  purposes.  They 
have  been  used  in  theatrical  productions,  but  the 
greatest  drawback  is  the  difficulty  of  obtaining  an 
illuminant  emitting  rays  of  short  wave-lengths  (which 
are  the  most  effective  in  exciting  phosphorescence) 


COLORED   MEDIA  341 


in  sufficient  intensities.  The  bare  carbon  arc  and 
the  quartz  mercury  arc  are  the  most  intense  excitants 
for  this  purpose  among  artificial  light  sources.  Lumi- 
nous calcium  sulphide,  sometimes  known  as  Balmain's 
paint,  is  cheap  and  active  and  emits  phosphorescent 
light  of  fairly  long  duration.  It  forms  the  basis  of 
several  cheap  though  not  highly  satisfactory  phos- 
phorescent paints. 

Phosphorescent  oil  paints  can  be  made  by  using 
pure  linseed  oil  instead  of  the  varnish  which  is  ordi- 
narily used  in  phosphorescent  paints.  For  artists' 
paints  the  varnish  should  be  replaced  by  pure  poppy 
oil.  Phosphorescent  material  can  be  applied  to  cloth 
and  paper  by  omitting  the  varnish,  mixing  the  powder 
in  water,  and  applying  this  paste  in  a  convenient 
manner.  For  applying  to  glass  or  porcelain,  the 
varnish  is  replaced  by  Japanese  wax  in  a  slightly 
greater  quantity,  and  olive  oil  is  added.  These  mix- 
tures can  be  fired  successfully  when  air  is  excluded. 
Water  glass  (sodium  silicate)  is  a  satisfactory  pro- 
tecting agent  for  such  applications. 

87.  Miscellaneous  Notes.  —  For  purely  decorative 
effects  of  a  temporary  nature  some  of  the  colored 
metallic  salts  that  crystallize  when  the  solvent  is 
evaporated  are  quite  useful.  For  instance,  if  a  sat- 
urated solution  of  potassium  bichromate  be  added  to 
a  rather  concentrated  aqueous  solution  of  gelatine, 
and  this  mixture  be  flowed  while  hot  upon  a  level 
plate  glass,  on  cooling  it  forms  a  yellow  diffusing 
filter  of  crystalline  structure.  Such  screens  do  not 
have  a  wide  application;  nevertheless  they  can  be 
used  for  temporary  decorative  purposes.  A  weak 
solution  of  potassium  bichromate  can  be  used  in 
gelatine  without  crystallizing  or  drying.  The  greenish 
tinge  of  this  yellow  can  be  overcome,  if  desirable, 


342  COLOR  AND   ITS  APPLICATIONS 

by  an  addition  of  a  slight  quantity  of  a  dilute  solu- 
tion of  a  red  or  pink  dye. 

The  air  brush  is  a  useful  instrument  for  the  appli- 
cation of  liquid  colorings,  especially  when  the  pig- 
ments do  not  readily  dissolve  in  lacquers.  Lamps 
and  other  objects  can  be  readily  colored  by  immer- 
sion in  a  colored  lacquer,  but  this  is  not  a  very  satis- 
factory procedure  when  the  coloring  matter  is  merely 
held  in  suspension.  By  means  of  an  air  brush  any 
colored  solution  can  be  readily  applied  to  an  object 
with  a  fair  degree  of  uniformity.  Perhaps  the  most 
discouraging  factor  in  the  production  of  colored 
lighting  effects  is  the  lack  of  permanent  blue  and 
blue-green  pigments  that  will  readily  dissolve  in  a 
satisfactory  lacquer.  Prussian  blue  and  cobalt-blue 
are  quite  permanent,  but  insoluble  in  common  lac- 
quers. These  can  be  successfully  applied  by  means 
of  an  air  brush  when  they  are  held  in  suspension  in 
a  lacquer  of  thin  varnish.  By  occasionally  diverting 
the  flow  of  air  through  the  liquid  such  insoluble  pig- 
ments can  be  kept  in  suspension  in  the  binding  solu- 
tion. For  this  class  of  work  a  small  motor  operated 
from  two  or  three  dry  cells  or  a  small  transformer  and 
equipped  with  a  vertical  stirring  rod  is  exceedingly 
useful. 

Pigments  can  be  readily  tested  for  durability  by 
placing  them  on  strips  of  glass  and  partially  covering 
them  with  glass.  These  should  be  exposed  to  sun- 
light or  to  the  radiation  from  an  arc  lamp,  keeping 
part  of  the  pigment  covered.  The  exposed  portions 
should  include  both  the  unprotected  portion  and  that 
protected  by  the  cover  glass.  Another  convenient 
method,  depending  of  course  upon  the  final  uses  to 
which  the  pigments  or  lacquers  are  to  be  put,  is  found 
in  applying  them  directly  to  incandescent  lamp  bulbs. 


COLORED   MEDIA  343 


The  lamps  should  be  dyed  in  pairs,  and  one  should 
be  preserved  while  the  other  be  operated  on  normal 
or  slightly  above  normal  voltage.  If  the  pigments 
are  eventually  to  be  exposed  to  the  weather,  the  tests 
should  be  made  out  of  doors. 

These  are  a  few  data  that  have  arisen  in  ex- 
perimental work  in  the  study  and  application  of  the 
science  of  color  and  in  the  production  of  various  color 
effects  which  may  prove  helpful  to  those  interested  in 
color.  See  next  chapter. 

REFERENCES 

M.  Toch,  Materials  for  Permanent  Painting,  1911;  Chemistry 
and  Technology  of  Mixed  Paints. 

E.  J.  Parry  and  J.  H.  Coste,  The  Chemistry  of  Pigments. 

F.  S.  Hyde,  Solvents,  Oils,  Gums,  and  Waxes. 

C.  H.  Hall,  Chemistry  of  Paint  and  Paint  Vehicles. 
W.  R.  Mott,  Paint  and  Dye  Testing,  Trans.  Amer.  Electrochem. 
Soc.  1915. 


CHAPTER   XVII 
CERTAIN  PHYSICAL  ASPECTS   AND   DATA 

88.  A  perusal  of  the  literature  on  colored  media  and 
a  general  acquaintance  with  color  industries  has  led  to 
the  conclusion  that  the  chemistry  of  such  substances 
greatly  dominates  the  physics  in  color-technology.  In 
fact,  much  of  the  physics  of  color  is  so  little  used  in  some 
of  these  activities  that  it  is  either  not  generally  under- 
stood by  color-technologists  or  its  value  is  underestimated. 
Spectral  analyses  —  the  quantitative  determinations  of 
the  spectral  characteristics  of  colored  materials  —  pro- 
vide the  foundations  for  many  important  aspects  of  color- 
technology  and  without  such  data  some  work  is  con- 
ducted more  or  less  blindly.  With  such  data  and  those 
derived  from  less  analytical  methods,  many  interesting 
facts  of  color-technology  can  be  bared  and  various 
factors  can  be  determined  which  are  unapproachable 
from  the  viewpoint  of  chemistry  or  from  ordinary  visual 
inspection.. 

Of  the  various  methods  of  analyzing  color,  that  of 
the  spectrophotometer  is  the  most  analytical  and  it 
provides  data  of  far  greater  usefulness  hi  the  physics 
of  color  than  the  data  which  are  yielded  by  any  of 
the  other  methods.  By  this  method  the  reflection-  (or 
transmission-)  factors  of  the  coloring  media  are  de- 
termined for  radiant  energy  of  all  wave-lengths  in  the 
visible  spectrum.  When  these  are  plotted  we  have  the 
spectral  reflection  (or  transmission)  curves  for  the 
visible  spectrum.  By  the  same  method  the  spectral 
character  of  an  illuminant  may  be  obtained.  By  multi- 

344 


CERTAIN  PHYSICAL  ASPECTS   AND   DATA          345 

plying  the  relative  energy-values  of  the  various  wave- 
lengths of  any  illuminant  by  the  corresponding  visi- 
bilities of  radiation,  the  spectral  luminosity-distribution 
curves  are  obtained  for  the  given  illuminant.  These 
latter  will  vary  with  the  illuminant  and  are  often  of 
greater  importance  than  the  spectral  energy-distribution 
curves  from  a  visual  viewpoint.  It  is  obvious  that  by 
multiplying  corresponding  spectral  values,  the  spectral 
energy-distribution  and  luminosity-distribution  curves 
of  any  colored  medium  maybe  readily  obtained  for  any  illu- 
minant. Such  data  and  their  uses  will  be  presented  later. 

Owing  to  the  indefiniteness  and  limitations  of  the 
data  yielded  by  most  of  these  so-called  colorimetric 
methods  and  the  difficulties  attending  the  use  of  the 
monochromatic  colorimeter  at  present,  this  chapter  will 
be  confined  almost  entirely  to  spectrophotometric  data 
and  their  uses.  Many  instances  arise  when  the  degree 
of  absorption  for  ultra-violet  and  infra-red  rays  is  of 
interest.  The  former  can  be  determined  readily  by 
spectrophotography  and  the  latter  by  means  of  such 
energy-measuring  instruments  as  the  bolometer  or  ther- 
mopile. 

89.  Types  of  Colored  Media.  —Three  classes  of 
colored  media  will  be  represented  and  discussed,  namely, 
pigments,  dyes,  and  vitrifiable  colors  or  colored  glasses. 
Pigments  are  distinguished  from  dyes  by  their  insolu- 
bility in  their  vehicle,  while  dyes  are  soluble.  This 
distinction  may  appear  arbitrary,  especially  hi  some  cases, 
however,  it  is  employed  to  some  extent  and  is  a  con- 
venient classification.  Pigments  may  be  distinguished 
from  paints  in  that  the  latter  are  pigments  in  a  vehicle 
or  medium.  Vitrifiable  colors  are  those  which  impart 
color  to  glass  and  to  similar  substances.  Among  pig- 
ments are  found  two  general  classes;  one  in  which 
each  particle  is  homogeneous  and  the  other  in  which 


346  COLOR  AND  ITS  APPLICATIONS 

a  colorless  base  has  been  colored  by  depositing  coloring 
matter  upon  it.  Colored  media  vary  in  many  physical 
characteristics  such  as  opacity,  fineness,  and  refractive- 
index,  and  they  may  be  considered  as  varying  hi  'color- 
ing power.' 

The  color  of  a  pigment  in  a  finely  divided  state, 
whether  the  particles  are  separated  by  air  or  by  a  vehicle, 
is  due  to  innumerable  selective  reflections  from,  and 
transmissions  through,  the  minute  particles.  If  the 
powdered  pigment  is  given  a  smooth  surface  by  pres- 
sure it  does  not  appear  as  pure  in  color  as  when  it  is 
loosely  packed  because  in  the  latter  case  a  greater 
proportion  of  the  incident  radiant  energy  is  able  to 
penetrate  more  deeply  into  the  body  and  becomes  colored 
by  selective  reflections  and  transmissions.  Radiant 
energy  is  regularly  reflected  from  even  the  small  sur- 
faces of  the  particles  of  pigment  and  in  those  cases  where 
the  minute  areas  of  surface  are  properly  oriented,  this 
regularly  reflected  light  does  not  find  its  way  further 
into  the  pigment  but  is  reflected  practically  unaltered 
in  spectral  character  as  compared  to  that  energy  which 
penetrates  further  into  the  mass.  Thus,  there  is  always 
reflected  from  pigments  some  radiant  energy  which  is 
practically  unchanged  in  spectral  character  which  ac- 
counts partly  for  the  general  lack  of  purity  of  the  colors 
of  pigments.  It  is  seen  that  the  character  of  the  surface 
is  important.  Furthermore,  the  refractive-indices  of 
the  pigment  and  of  the  vehicle  (air  in  the  case  of  dry 
powders)  are  of  importance  because  the  amount  of  light 
regularly  reflected  from  a  surface  is  dependent  upon 
these  refractive-indices.  A  careful  study  of  the  influence 
of  the  vehicle  upon  the  color  of  a  paint  leads  to  inter- 
esting data  from  this  viewpoint  alone. 

Careful  observation  will  reveal  the  influence  of  the 
porosity  of  a  pigment-surface  upon  its  color.  An  excel- 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA         347 

lent  example  for  the  purpose  of  illustration  here  is  the 
color  of  a  white  cotton  fabric  compared  with  that  of 
a  white  silk  fabric  after  both  have  been  soaked  in  the 
same  dye-solution.  In  Fig.  130  are  shown  reproductions 
of  microphotographs  of  white  cotton  and  silk  fabrics 
as  photographed  against  a  black  background.  It  is 
seen  that  the  silk  is  more  transparent  than  the  cotton 
fibers;  in  fact,  the  cotton  fibers  are  merely  translucent 
as  compared  with  the  transparency  of  silk  fibers.  The 
latter  permit  the  radiant  energy  to  penetrate  more 
deeply,  hi  general,  than  the  cotton  fibers;  in  other 
words,  the  cotton  fibers  by  diffuse  reflection  turn  the 
energy  backward  before  it  has  penetrated  very  deeply. 
For  this  reason  the  silk  fabric  appears  of  a  purer  color 
compared  with  that  of  the  cotton  fabric  dyed  in  the 
same  solution,  the  result,  hi  the  case  of  the  silk,  being 
similar  to  that  which  would  have  been  obtained  with  the 
cotton  if  the  latter  had  been  dyed  in  a  more  concen- 
trated solution  of  the  dye. 

In  a  manner  similar  to  the  case  of  pigments  the 
solvent  appears  to  have  certain  influences  upon  the  color 
of  a  solution  of  a  dye  although  this  subject  has  not  been 
thoroughly  studied.  The  substance  upon  which  a  dye 
has  been  deposited  by  immersion  is  also  of  importance 
in  spectral  analysis  as  is  indicated  by  the  case  of  dyeing 
cotton  and  silk  fibers,  Fig.  130.  The  transmission- 
factor  of  a  dye-solution  is  a  simple  logarithmic  function 
of  the  depth  of  a  given  solution  or  of  its  concentration, 
but  this  relation  varies  with  the  wave-length,  in  general 
in  no  definite  relation  between  wave-length  and  spectral 
transmission-factor.  For  this  reason  no  simple  relation 
between  total  transmission  and  depth  or  concentration 
can  be  established.  Such  values  of  total  transmission 
must  be  determined  by  direct  measurement  or  by  in- 
tegration, as  will  be  discussed  later. 


Fig.  130.  — Cotton. 


Fig.  130.  — Silk. 


CERTAIN  PHYSICAL  ASPECTS  AND   DATA         349 

Colored  glasses  can  be  treated  much  in  the  same 
manner  as  dye-solutions.  A  given  concentration  of 
coloring  material  in  a  glass,  that  is,  a  given  colored 
glass,  apparently  obeys  the  same  law  relating  thickness 
and  transmission-factor  for  a  given  wave-length  as  a 
dye-solution.  However,  it  is  not  established  that  the 
introduction  of  various  amounts  of  the  coloring  material 
(generally  metallic  oxides)  results  in  corresponding  con- 
centration as  would  be  true  in  the  case  of  dyes.  In  glass 
there  is  more  or  less  chemical  action  and  the  uncertain 
conditions  of  melting  make  this  point  difficult  to 
decide. 

The  physics  of  the  process  by  which  glasses  are 
colored  by  means  of  metallic  compounds  is  not  wholly 
clear.  There  are  many  chemical  analogies  which  are 
of  interest  for  their  parallelism  to  the  colors  imparted 
to  glasses  by  the  metals  in  different  states  but  the  reasons 
for  the  appearance  of  the  colors  cannot  be  considered 
as  being  thoroughly  established.  Garnett1  has  pre- 
sented a  very  interesting  discussion  of  the  colors  ex- 
hibited by  certain  glasses  in  which  metallic  oxides  had 
been  incorporated.  It  is  a  common  supposition  that  the 
colors  of  certain  glasses,  such  as  gold  red  glasses,  are 
due  to  the  presence  of  very  minute  particles  of  metal. 
Solutions  of  some  metals  exhibit  colors  which  are  often 
exhibited  by  colored  glasses  in  which  the  same  metals 
have  been  introduced.  Siedentopf  and  Szigmondy,  by 
powerfully  illuminating  specimens  of  colored  glass  and 
colored  colloidal  solutions  of  metals  obliquely,  or  at 
right-angles  to  the  line  of  sight,  were  able  to  detect  the 
presence  of  the  metallic  particles.  Garnett's  work  ex- 
plained some  of  their  observations. 

It  is  commonly  considered  that  metals  color  glass 
in  two  ways,  one  by  being  in  a  state  of  true  solution  in 
the  glass  and  the  other  by  being  in  a  colloidal  state. 


350  COLOR  AND  ITS  APPLICATIONS 

An  example  of  the  former  is  copper  blue-green  glass  and 
of  the  latter,  gold  red  glass. 

In  dealing  with  the  physics  of  colored  media  from  the 
viewpoint  of  the  physicist,  one  cannot  avoid  the  con- 
clusion that  there  is  a  wide  application  of  physics  to  color- 
technology  in  many  directions  quite  unexplored. 

90.  Pigments.  —  In  presenting  data  which  it  is  hoped 
will  be  of  direct  use  to  others,  only  those  colored  media 
have  been  selected  which  are  thought  to  be  fairly  con- 
stant in  composition  and  representative.  The  spectral 
reflection-factors  of  a  group  of  dry  powdered  pigments, 
commonly  used  in  the  paint  industries  and  which  from 
general  observation  appear  representative,  were  de- 
termined by  means  of  the  spectrophotometer  and  the 
data  are  presented  in  Table  XXII.  The  light  was  re- 
flected from  a  thick  layer  of  the  powder,  the  surface 
being  gently  smoothed  by  means  of  a  sheet  of  plane 
glass.  Whites  and  blacks  have  been  omitted  but  these 
are  by  no  means  always  neutral  pigments.  Whites  are 
very  commonly  yellowish  and  blacks  (which  are  only 
approximately  black,  varying  in  reflection-factor  from 
0.02  to  0.1)  are  often  bluish  or  reddish.  Although  these 
departures  from  neutrality  are  not  relatively  great  they 
are  sufficient  to  be  detected  by  means  of  the  spectropho- 
tometer. Such  small  departures  are  readily  detected  by 
painting  the  inner  surface  of  a  box  with  such  a  sup- 
posedly neutral  pigment  and  by  viewing  a  white  surface 
indirectly  lighted  by  means  of  a  light-source  inside  the 
box.  The  visible  radiation  suffers  innumerable  re- 
flections (see  #  65)  from  the  walls  of  the  box  and  that 
which  illuminates  the  white  surface  is  therefore  much 
more  colored  than  the  pigment  would  appear  under  di- 
rect illumination.  The  spectral  reflection-factors  of  pig- 
ments are  more  difficult  to  obtain  than  the  transmission- 
factors  of  dyes  in  solution  because  in  the  former  case 


CERTAIN   PHYSICAL  ASPECTS  AND  DATA 


351 


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352 


COLOR  AND   ITS  APPLICATIONS 


more  or  less  energy  is  regularly  reflected  from  the 
particles  directly  into  the  instrument.  Care  must  be 
taken  to  avoid  placing  the  pigment  surface  in  such  a 
position  with  respect  to  the  slit  of  the  instrument  and 
to  the  light-source  that  an  undue  amount  of  regularly 
reflected  visible  radiation  enters  the  instrument.  The 
visible  radiation  which  is  thus  regularly  reflected  is 


B,  CMKOffC  YflLOI#(t1C6/(/fl) 

C,  A ffl '£ •  A/CAN 

D, 
C, 

f, 
G, 


\ 


L 


Fig.  131.  —  Pigments. 

practically  unchanged  hi  spectral  character  as  compared 
with  that  which  penetrates  into  the  interstices  of  the 
pigment  and  is  colored  by  innumerable  transmissions 
through,  and  reflections  from,  the  minute  particles  of 
pigment.  At  any  angle  some  of  the  energy  is  regularly 
reflected  from  the  minute  portions  of  the  surfaces  of 
the  particles  which  are  properly  oriented.  This  accounts 
partly  for  the  general  lack  of  purity  of  the  colors  of 
'opaque*  pigments.  The  data  of  Table  XXII  are  plotted 
in  Figs.  131  and  132. 

Spectral  analyses  in  the  ultra-violet  and  infra-red 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA 


353 


regions  are  often  of  interest  in  general  color-technology. 
In  the  former  case  spectrophotography  is  the  simplest 
method  of  attack  although  the  procedure  is  a  tedious 
one  if  high  accuracy  is  desired.  It  is  necessary  to  es- 
tablish photographic  density  and  pigment-illumination 
(or  exposure)  relations  for  various  wave-lengths  in  order 
to  obtain  the  reflection-factors  for  radiant  energy  of 


0.44M 


Fig.  132.  —  Pigments. 

various  wave-lengths.  Besides  this  the  ordinary  pre- 
cautions of  photographic  procedure  must  be  taken. 
Another  possible  method  is  that  which  involves  the  use 
of  the  photo-electric  cell.  No  systematic  data  on  pig- 
ments in  the  ultra-violet  region  have  been  obtained 
so  none  will  be  presented,  although  ofttimes  it  has  been 
necessary  to  investigate  this  region  for  a  particular  pig- 
ment. It  is  well  to  recognize  the  importance  of  such 
analyses  in  cases  involving  ultra-violet  light.  An  ex- 
cellent example  is  zinc  white  which  absorbs  ultra-violet 
energy  quite  freely. 


354  COLOR  AND  ITS  APPLICATIONS 

The  investigation  of  the  infra-red  region  requires 
a  more  elaborate  apparatus  although  in  many  cases 
where  total  energy-absorption  is  of  interest  this  can  be 
obtained  rather  easily  by  means  of  the  thermopile  or 
bolometer.  In  fact,  the  ordinary  radiometer  or  even 
the  thermometer  covered  with  a  pigment  yields  data 
which  have  some  uses  in  practice.  Coblentz2  has 
published  interesting  data  on  the  reflection-factors  of 
various  substances  for  infra-red  and  visible  radiation 
of  several  wave-lengths.  Among  the  substances  which 
he  studied  were  a  number  of  pigments.  The  reflection- 
factors  of  white  pigments  for  energy  of  wave-lengths 
4.4ju  varied  from  about  0.1  to  0.4  and  at  8.8/z  and  24/z 
were  considerably  lower.  These  data  especially  empha- 
size the  localized  nature  of  absorption-bands  as,  for 
example,  cobalt  oxide  is  a  better  reflector  of  long-wave 
energy  than  zinc  oxide,  yet  for  visible  rays  it  possesses 
an  extremely  lower  reflection-factor  than  zinc  oxide. 
Lead  oxide  is  a  much  more  efficient  reflector  of  long-wave 
energy  than  zinc  oxide,  magnesium  carbonate  and  other 
white  pigments.  The  importance  of  the  infra-red  analyses 
is  apparent  in  many  practical  activities.  Coblentz  has 
pointed  out  that  a  pigment  which  has  a  low  reflection- 
factor  for  energy  of  wave-lengths  in  the  region  of  8ju 
to  9/z  is  a  better  house  paint  in  hot  climes  because  it 
re-radiates  maximally  in  this  region  where  the  maximum 
radiation  from  bodies  of  temperatures  from  20°  to  25°  C. 
is  found.  If  the  paint  has  a  high  reflection-factor  for 
visible  rays  it  thus  minimizes  the  heating  effect  of  the 
incident  energy.  Such  a  combination  is  quite  desirable 
in  minimizing  the  heating  effect  of  solar  rays.  This  is 
merely  one  example  of  a  vast  number  of  interesting 
problems  which  could  be  met  with  more  intelligence  if 
spectral  analyses  were  available. 

91.  Some  Optical  Properties  of  Pigments.  —  In  con- 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA         365 

sidering  the  optical  properties  of  a  painted  surface  it  is 
necessary  to  distinguish  between  a  pigment  and  a  paint. 
The  former  and  its  vehicle  constitute  the  paint  and  the 
optical  properties  of  a  painted  surface  depend  not  only 
upon  those  of  the  pigment  but  also  upon  the  vehicle  and 
the  surface  covered.  H.  E.  Merwin 3  has  made  interest- 
ing studies  of  these  properties. 

Most  pigments,  with  the  exception  of  lakes,  consist 
of  minute  crystals  and  the  color  commonly  varies  with 
the  direction  of  the  passage  of  light  through  a  crystal. 
Therefore,  the  shape  of  these  crystals  influences  the 
value  of  a  pigment.  The  transmission-  (and  absorption-) 
bands  of  pigment  crystals  are  rather  wide  and  shallow 
and  small  grains  are  more  transparent  than  large  ones 
of  the  same  material.  For  this  reason,  a  pigment  con- 
sisting of  small  grains  is  generally  brighter  than  one  of 
large  grains.  Small  grains  may  be  considered  to  have 
diameters  of  the  order  of  magnitude  of  IM  and  large 
ones  of  the  order  of  10/z.  Usually  the  diameter  of  grains 
of  colored  pigments  lies  between  0.5/z  and  10/*. 

The  coloring  power  of  a  pigment  generally  increases 
as  the  size  of  the  grain  decreases  but  there  is  no  definite 
relation  covering  different  substances.  For  a  given 
amount  of  pigment  it  is  obvious  that  the  total  amount 
of  surface  exposed  to  intercept  light  increases  inversely 
as  the  square  of  the  diameter  of  the  grains,  although 
the  ability  of  a  grain  to  alter  transmitted  light  in  any 
direction  increases  more  slowly  than  the  diameter. 

Merwin  considered  four  classes  of  colored  pigments 
with  regard  to  their  adaptability  to  the  making  of  tints 
and  shades.  They  are  as  follows: 

a.  Colored  grams  are  chiefly  of  such  size  that  if 
closely  packed  in  a  single  layer  they  would  transmit 
(or  diffuse  and  transmit)  a  clear  tint  (say  roughly  40 
to  60  per  cent,  white).  From  5  to  20  such  layers  would 


356  COLOR  AND  ITS  APPLICATIONS 

produce  a  full  color.  Either  clear  tints  or  pure  shades 
can  be  made  from  such  a  pigment.  Examples:  chrome 
orange,  chrome  yellow,  verdigris,  ultramarine  blue. 

b.  Grains  are  so  transparent  that  white  light  after 
traversing  many  layers  of  grains  still  contains  a  good 
deal  (20  per  cent,  or  more)  white.    Such  a  pigment  can 
be  used  in  making  clear  tints  but  not  pure  shades.    Ex- 
amples: barium  yellow,  basic  copper  carbonate,  stron- 
tium yellow. 

c.  A  single  layer  of  grains  absorbs  several  per  cent, 
of  the  characteristic  hue,  and  other  hues  almost  com- 
pletely.   Pure  shades  and  dull  tints  may  be  made  from 
such  a  pigment.    Examples :  vermilion,  scarlet  chromate, 
Harrison  red,  chrome  green. 

d.  Single   grains   absorb    several   per    cent,   of  the 
characteristic  hue   and   even  several  layers   of  grains 
do  not  absorb  other  hues  completely.    When  darkened 
by  a  black  pigment  dull  shades  result,  and  when  lightened 
by  a  white  pigment  dull  tints  are  formed.     Examples: 
Naples  yellow,  some  Dutch  pinks  and  yellow  ochres. 

In  the  last  two  classes  diffusing  power  determines 
to  some  extent  and  absorbing  power  to  a  greater  extent, 
what  range  of  pure  shades  can  be  obtained. 

Vehicles  when  dried  have  refractive-indices  in  the 
neighborhood  of  1.5  and  this  indicates  that  the  amount 
of  light  regalarly  reflected  from  a  smooth  surface  of  a 
vehicle  is  about  four  per  cent.  A  substance  to  be  most 
effective  as  a  pigment  should  have  a  high  refractive- 
index  for  the  hue  it  most  freely  transmits.  The  re- 
fractive-index varies  considerably  in  the  neighborhood 
of  an  absorption-band,  being  greater  on  the  long-wave 
side  than  on  the  short-wave  side.  This  is  a  reason  for 
the  greater  refractive-indices  usually  exhibited  by  yellow, 
orange,  and  red  pigments  than  by  blue  and  violet.  Of 
course,  the  refractive-index  of  a  lake  is  largely  deter- 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA         357 

mined  by  the  base  and  is  usually  comparatively  low. 
If  the  refractive-index  of  a  pigment  closely  matches 
that  of  the  vehicle,  the  former  will  diffuse  very  little 
light.  Such  a  pigment  would  ordinarily  be  mixed  with 
one  of  higher  refractive-index  which  will  diffuse  the  light. 

Obviously,  a  black  pigment  to  appear  black  in  a 
dried  vehicle  should  have  the  same  refractive-index 
as  that  of  the  vehicle  and  it  must  absorb  all  the  light 
incident  upon  it.  In  the  dry  state  surrounded  by  air  the 
pigment  particles  will  reflect  some  light  regardless  of 
their  absorbing  power.  Even  when  the  refractive- 
indices  of  pigment  and  vehicle  are  equal,  there  is  re- 
flected directly  from  the  surface  of  the  paint  about 
4  per  cent,  of  the  incident  light.  To  overcome  this, 
light-traps  such  as  possessed  by  a  velvet  may  be  pro- 
vided. Ivory  black  is  an  excellent  black  because  its 
refractive-index  is  nearly  the  same  as  that  of  oil  or 
varnish. 

From  the  foregoing  consideration  it  is  obvious  that 
a  high  refractive-index  is  essential  to  a  white  pigment. 
The  grains  should  be  fine  and  there  should  be  no  selec- 
tive scattering  of  light  of  various  wave-lengths.  The 
burning  vapor  of  metallic  zinc  produces  very  fine  grains 
of  zinc  oxide.  These  are  less  than  I/*  in  diameter. 
Most  of  the  zinc  oxides  contain  enough  fine  grains  less 
than  IM  in  diameter  to  give  a  bluish  tint  to  paints  by  virtue 
of  the  selective  scattering  of  light  of  the  shorter  wave- 
lengths. 

92.  Some  Applications  of  Spectral  Analyses  of  Pig- 
ments. —  The  chief  use  of  data  derived  from  such 
spectral  analyses  is  that  of  establishing  the  spectral 
character  of  the  pigment.  The  general  value  of  such  data 
needs  no  defense,  for  it  is  the  actual  foundation  of  the 
pigment  as  a  coloring  material.  Its  purity  is  thus  estab- 
lished; its  influence  in  color-mixture  may  be  predicted; 


358  COLOR  AND  ITS  APPLICATIONS 

the  purity  or  desirability  of  a  color  resulting  from  various 
mixtures  of  pigments  whose  spectral  analyses  are  avail- 
able may  be  predetermined;  and  in  many  ways  such 
data  are  useful.  It  is  quite  beyond  the  scope  of  a  single 
chapter  to  discuss  all  the  physical  uses  of  such  data, 
besides  it  is  the  intention  to  confine  the  discussion 
chiefly  to  aspects  which  are  likely  to  be  less  commonly 
appreciated.  For  the  latter  purposes  other  data  such 
as  the  spectral  energy-distribution  in  illuminants  and 
the  visibility  of  radiation  of  various  wave-lengths  are 
necessary,  therefore  Table  XXIII  is  presented.  The 
relative  energy-values  at  various  wave-lengths  are  given 
for  four  illuminants  which  represent  nearly  the  extremes 
commonly  encountered  from  the  viewpoint  of  color. 
In  the  last  column  are  presented  the  visibility  data4 
standardized  in  the  1918  report  of  the  Nomenclature 
and  Standards  Committee  of  the  Illuminating  Engineer- 
ing Society.  There  is  no  exact  agreement  as  yet  among 
investigators  regarding  the  visibility  of  radiation  of 
different  wave-lengths,  however,  the  data  are  sufficiently 
well  established  for  the  present  purpose.  On  multi- 
plying each  ordinate  of  a  spectral  energy  distribution 
curve  of  an  illuminant,  pigment,  dye,  etc.,  by  the  cor- 
responding value  of  visibility,  the  resultant  data  yield 
the  spectral  luminosity-distribution  of  the  illuminant, 
pigment,  dye,  etc.  Thus,  from  the  spectral  energy  and 
visibility  data  the  relative  spectral  luminosity-values  can 
be  determined.  On  integrating  the  areas  of  the  spectral 
luminosity  curves,  the  relative  total  luminosity-values 
of  colored  media  and  of  illuminants  can  be  obtained  and 
by  dividing  the  area  of  one  of  the  former  by  the  area  of 
one  of  the  latter,  the  reflection-factor  of  the  particular 
colored  medium  is  obtained  for  the  particular  illuminant. 
Thus  by  computation,  the  reflection-factors  of  colored 
media  can  be  obtained  without  any  of  the  difficulties  and 


CERTAIN  PHYSICAL  ASPECTS  AND   DATA 


359 


TABLE  XXIII 

Spectral  Energy-Distribution  in  Common  Illuminants  and  the  Visibility 

of  Radiation 


Wave- 
length 

Blue  sky 

Noon  sun 

Tungsten 
(vacuum) 
Incandescent 
Lamp 
7.9  lumens 

Tungsten 
(gas-filled) 
Incandescent 
Lamp 
22  lumens 

Visibility  of  radiation* 

Relative  to 
that  at  556MM 

Absolute 
(Lumens  per 
watt) 

watt 

watt 

0.40/i 

170 

67 

9 

15 

0.0004 

0.0000006 

.41 

177 

72 

95 

16.5 

.0012 

.0000018 

.42 

181 

75 

10.5 

19 

.0040 

.0000060 

.43 

185 

79 

12 

23 

.0116 

.000017 

.44 

186 

83 

15 

26.5 

.023 

.000034 

.45 

187 

84.3 

16.7 

30 

0  038 

0.000057 

.46 

185 

88 

20 

33.7 

.060 

.000090 

.47 

180 

91 

23.5 

38 

.091 

.000136 

.48 

173 

92 

27 

42.6 

.139 

.000208 

.49 

162 

92.5 

32.7 

47 

.208 

.000312 

.50 

157 

95 

37  5 

52 

0  323 

0.00048 

.51 

146 

96 

42.6 

56  5 

.484 

.00073 

.52 

140 

97 

49 

62 

.670 

.00100 

.53 

132 

98 

54.9 

67 

.836 

.00125 

.54 

127 

99 

62.1 

72.5 

.942 

.00142 

.55 

120 

99 

68.6 

78 

0  993 

0.00149 

.56 

115 

100 

76 

83 

.996 

.00149 

.57 

108 

100 

83.4 

88 

.952 

.00143 

.58 

104 

101 

91 

94 

.870 

.00130 

.59 

100 

100 

100 

100 

.757 

.00114 

.60 

97 

100 

108 

105 

0  631 

0.00095 

.61 

93 

100 

117 

111 

.503 

.00075 

.62 

90 

99 

126 

116 

.380 

.00057 

.63 

87 

98.5 

136 

121.5 

.262 

.00039 

.64 

85 

98 

146 

126 

.170 

.00025 

.65 

82 

97.1 

157 

131 

0.103 

0.000154 

.66 

80 

96 

167 

135 

.059 

.000089 

.67 

77 

95.5 

179 

140 

.030 

.000045 

.68' 

76 

94 

189 

143 

.016 

.000024 

.69 

72.5 

93.5 

202 

147.5 

.0081 

.0000122 

.70 

71 

91.7 

212 

151 

0.0041 

0.0000061 

.71 

69.6 

90 

223 

153.5 

.0021 

.0000031 

.72 

68 

88 

235 

156 

.0010 

.0000015 

*  Standardized  in  1918  Report  of  Committee  on  Nomenclature  and  Standards 
of  I.E.S. 

uncertainties  of  color-photometry,  for  these  have  been 
involved  in  the  determination  of  the  visibility  data. 
Such  computations  are  found  to  yield  results  quite  in 


360 


COLOR  AND   ITS  APPLICATIONS 


agreement  with  those  obtained  by  direct  measurement 
of  reflection-  (or  transmission-)  factor.  In  fact,  this 
method  appeals  very  strongly  to  the  author,  especially 
because  the  spectral  analyses  should  be  available  for 
many  other  reasons  so  that  reflection-  and  transmission- 
factors  would  be  by-products. 

The  spectral  luminosity-distributions  of  the  visible 
radiation  reflected  from  pigments  whose  spectral  re- 


sf- 

£-  CHROMC  rf 

C  - 

£- 
£- 


fuami 


1£  6 


1 


x\ 


Q44M 


Fig.  133.  —  Pigments. 

flection-factor  distributions  are  shown  in  Figs.  131  and 
132  and  in  Table  XXII  are  presented  in  Figs.  133  and  134. 
These  may  also  be  considered  as  the  spectral  reflected- 
energy  distributions  for  an  imaginary  illuminant  of 
uniform  spectral  energy-distribution.  Incidentally,  the 
light  from  the  noonday  sun  approaches  this  ideal  fairly 
closely  as  seen  by  Table  XXIII  for  in  this  table  the 
energy-values  of  this  ideal  illuminant  would  be  100  for 
all  wave-lengths  in  order  to  be  directly  comparable 
with  the  other  illuminants. 


CERTAIN  PHYSICAL  ASPECTS   AND   DATA 


361 


93.  Re  flection- factor    of   Pigments.  —  In    order    to 
cover  the  general  case  more  accurately  much  of  the  fore- 
going discussion  will  be  expressed  mathematically  but, 
for  the  sake  of  clearness,  reference  will  be  made  to 
these  various  curves  in  Fig.  135  for  a  specific  case. 
/     =  Spectral   energy-distribution    of 
an  illuminant  (tungsten  fila- 
ment at  7.9  lumens  per  watt). 


J  -  fl/itr  J//VWX 
A"-  BufiHr  J/^/v/Vx* 

L-  fHOIJN  REO 

//-  COBALT Buse 


M         JO      J£ 


Letter* 


J\ 

V 
K\ 

LI 
P 


Fig.  134.  —  Pigments. 

=  Energy-value  of  the  illuminant  at 

any  wave-length,  \. 
=  Visibility  curve. 
=  Visibility-value    for    energy    of 

wave-length,  A. 
=  Spectral  luminosity-distribution 

of  illuminant  /. 
=  Spectral  reflection-factor  distri- 

bution of  a  pigment  (light 

chrome  yellow). 


362 


COLOR  AND   ITS  APPLICATIONS 


R\  =  Reflection-factor  of  the  pigment 

for  energy  of  wave-length,  X. 

LP    =  Spectral    luminosity-distribution 

of  radiant  energy  reflected 

by  the  pigment. 

For  X  =  0.52M,  ad  =  7X,  af  =  KX,  ae  =  #x,  ac  =  KXJX, 
and  ab  =  R\KXJ\. 


150 


100 


50 


74 


(XOjj        44 


46 


v« 


j60  64 


.66         7* 


Fig.  135.  —  Analysis  of  a  pigment. 

K^Jxd\  =  Area  enclosed  by  L/,  which  is  propor- 
tional to  the  total  luminous  flux  £,  received  by  the 
surface  between  limits  Xi  and  X2,  hence  is  equal  to  CE 
where  C  is  a  constant  of  proportionality.  If  the  total  is 
desired,  the  limits,  Xi  and  X2,  are  respectively  the  limits 
of  the  visible  spectrum  which  for  most  practical  cases 
may  be  taken  as  0.4^  and  0.7ju. 


R\K\J\d\  =  Area  enclosed  by  LPj  which  is  pro- 
portional  to  the  total  luminous  flux,  E' ',  reflected 
by  the  surface  (pigment  P)  and  is  equal  to  CE' . 


CRETAIN   PHYSICAL    ASPECTS  AND  DATA         363 

If  energy  is  of  interest  instead  of  luminosity,  K\  is 
eliminated  and  the  limiting  wave-lengths  Xi  and  X2  are 
given  the  desired  values. 


— £-—   =  -£  =  R  =  the  reflection-factor  of  the 

/K  J  d X  pigment  P  for  the  illuminant  /, 

and  Xi  and  X2  are  respectively  the 
wave-lengths  at  the  limits  of  the 

visible  spectrum.  These  limits  could  be  expressed  as  0 
and  oo  without  changing  the  result  because  beyond  the 
visible  spectrum  K\  is  zero. 

Many  useful  data  can  be  obtained  by  such  computa- 
tions when  the  spectral  energy-distributions  of  pigments 
and  of  illuminants  are  available.  These  computations 
can  be  made  for  a  sufficient  number  of  wave-lengths 
throughout  the  spectrum  and  the  relative  values  of  the 
integrals  can  be  obtained  by  means  of  a  planimeter 
from  the  plotted  curves  or  more  readily  by  summating 
the  computed  values. 

Similar  computations  have  been  made  for  the  group 
of  pigments  already  introduced  for  four  illuminants 
including  the  ideal  having  a  uniform  spectral  energy- 
distribution.  These  values  are  presented  in  Table  XXIV 
and  Fig.  136.  The  values  are  given  to  the  third  decimal 
place  not  with  the  belief  that  the  absolute  values  are 
determined  with  such  accuracy  but  to  show  the  differ- 
ences as  accurately  as  possible  obtained  by  this  method 
of  computation.  The  relative  values  are  perhaps  accurate 
to  the  third  place.  It  is  seen  that  the  reflection-factor 
for  a  given  pigment  is  not  constant  (see  #  42)  but 
varies  with  the  illuminant.  This  is  a  point  not  gener- 
ally appreciated  and  inasmuch  as  this  difference  exists 
the  suggestion  is  made  that,  for  general  purposes, 
reflection-factors  be  given  for  an  illuminant  of  uni- 


364 


COLOR  AND   ITS  APPLICATIONS 


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CERTAIN  PHYSICAL  ASPECTS  AND  DATA 


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COLOR  AND  ITS  APPLICATIONS 


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CERTAIN  PHYSICAL  ASPECTS  AND   DATA 


367 


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368 


COLOR  AND  ITS   APPLICATIONS 


form  spectral  energy-distribution.  In  cases  of  direct 
measurement  of  these  factors  the  clear  noon-day  sun 
sufficiently  approaches  the  ideal  as  will  be  shown  shortly. 
In  cases  where  other  illuminants  are  used  these  should 
be  specified. 

The  measurement  of  reflection-factor  directly  is  by 
no  means  standardized  and  in  the  case  of  colored  pig- 


Fig.  136.  —  Pigments. 

ments  this  measurement  is  attended  with  many  diffi- 
culties such  as  the  distribution  of  luminous  flux  upon 
the  surface,  its  angular  position  with  respect  to  the 
photometer,  color-photometry,  etc.  A  discussion  of  this 
has  been  presented  elsewhere.5 

In  Table  XXIV  the  relative  reflection-factors  of  each 
pigment  by  itself  for  the  four  illuminants  are  presented, 
that  for  the  uniform  energy-spectrum  being  taken  as 
unity.  This  gives  a  better  idea  of  the  magnitude  of  the 
variation  of  the  reflection-factor  with  the  spectral  char- 
acter of  the  illuminant.  These  values  are  plotted  in 
Fig.  137  and  as  would  be  expected,  the  red  and  yellow 


CERTAIN   PHYSICAL  ASPECTS  AND   DATA 


369 


pigments  show  relatively  greater  reflection-factors  for 
tungsten  light  than  for  blue  sky-light  with  the  values 
for  sunlight  (circles)  lying  between.  It  is  interesting  to 
note  the  proximity  of  the  circles  to  unity  which  repre- 
sents the  relative  value  of  reflection-factor  in  each 
case  for  the  ideal  illuminant  having  an  uniform  spectral 
energy-distribution. 

The  effect  of  the  illuminant  upon  the  appearance  of 


RVO 


o    VOON  ~S 
----  Slue  St 


Fig.  137.  —  Pigments. 

the  color  is  shown  in  Fig.  138  using,  for  example,  ultra- 
marine blue  whose  spectral  energy-distribution  is  shown. 
The  spectral  luminosity-distributions  of  this  pigment 
for  the  different  illuminants  have  been  computed  for 
equal  total  amount  of  reflected  light  (enclosed  areas 
equal).  Thus  an  idea  of  the  appearances  of  the  color 
can  be  formed  or  conversely  the  reason  for  these  differ- 
ent appearances  under  the  three  illuminants  is  manifest. 
Incidentally,  it  is  seen  that  the  pigment  is  of  a  purer 
color  under  blue  skylight  than  under  either  of  the  other 
illuminants.  The  wave-length  of  maximum  luminosity 
is  0.495/z  and  0.54/z  respectively  for  the  skylight  and 
tungsten  light.  This  wave-length  of  maximum  lumi- 


370 


COLOR  AND   ITS   APPLICATIONS 


nosity  is  not  necessarily  the  dominant  hue  of  the  color 
as  analyzed  by  the  eye  or  by  the  monochromatic  color- 
imeter although  these  are  often  nearly  coincident. 

94.  Spectral  Analyses  of  Dye-solutions.  —  The  mix- 
ture of  dyes  is  governed  by  the  same  subtractive 
principles  of  color-mixture  as  the  mixture  of  pigments 
although  the  greater  number  of  dyes  and  the  more 


(MOM 


40  JK.  ffif  .60  Jt* 

Fig.  138.  —  Ultramarine. 

exacting  or  delicate  applications  of  dyes  in  industries,  in 
the  making  of  accurate  filters,  etc.,  make  their  spectral 
analyses  of  perhaps  more  importance  than  in  the  case 
of  pigments.  Certainly,  a  knowledge  of  the  spectral 
characteristics  of  dyes,  as  in  the  case  of  pigments,  makes 
for  an  ease  and  certainty  in  making  and  in  visualizing 
mixtures  which  cannot  be  enjoyed  without  such  data. 
It  is  beyond  the  scope  of  this  section  to  present  a  com- 
plete discussion  of  the  usefulness  of  spectral  analyses 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA          371 

of  dyes  or  to  present  the  spectral  analyses  of  all  the  dyes 
available;  however,  a  few  representative  analyses  of 
dyes  most  common  and  perhaps  most  reproducible 
should  be  of  value.  These  are  presented  in  the  following 
tables  roughly  classified  as  to  color.  The  highest  ac- 
curacy is  not  claimed  for  these  data  because  it  does  not 
appear  worth  the  effort  necessary  because  there  is  no 
indication  that  these  dyes  are  in  general  constant  in 
spectral  characteristics  as  obtained  from  tune  to  tune 
in  the  market.  For  the  same  reason  it  has  not  been 
considered  necessary  to  give  values  of  concentration. 
From  the  data  presented  in  the  tables  it  is  possible 
to  obtain  an  idea  of  the  spectral  characteristic  of  a  given 
dye-solution  for  any  depth  of  the  particular  concen- 
tration employed  and  also  for  any  relative  value  of  con- 
centration. In  other  words,  from  the  data  in  the  tables 
and  the  discussion  which  follows  it  is  possible  to  be 
guided  in  the  selection  of  dyes  for  many  purposes. 
For  the  study  of  a  dye-solution  throughout  an  entire 
range  of  depth  and  concentration  by  the  method  de- 
scribed later,  the  spectral  analysis  should  be  obtained 
as  accurately  as  possible.  In  all  cases  where  not 
indicated  otherwise,  the  solvent  was  distilled  water. 
The  dyes  were  obtained  from  various  well-known  com- 
mercial sources.  Among  the  solutions  will  be  found  a 
few  solutions  of  metallic  salts  which  are  incorporated 
for  their  usefulness  as  filters.  All  data  have  been  cor- 
rected for  surface  reflections  and  for  the  absorption  of 
the  glass  cell  by  the  method  of  substitution. 

In  Table  XXV  are  presented  the  spectral  analyses 
of  a  number  of  dye-solutions  commonly  classed  as  red 
although  many  are  purple.  The  sharpness  of  the  absorp- 
tion- or  transmission-bands  is  readily  visualized  from 
the  data  although  it  is  of  advantage  to  plot  the  data  in 
many  cases.  There  are  some  excellently  sharp  bands 


372  COLOR  AND  ITS  APPLICATIONS 

shown,  for  example,  that  of  eosine  of  moderate  con- 
centration. In  some  cases  spectral  analyses  for  two  con- 
centrations have  been  presented. 

In  Table  XXVI  spectral  analyses  of  a  number  of 
yellows  are  presented.  It  is  noteworthy  that  there  is  no 
known  dye  which  transmits  only  a  narrow  region  near 
spectral  yellow.  The  value  of  sharp  absorption-bands 
is  seen  when  a  fairly  monochromatic  filter  is  desired. 
For  instance  a  yellowish  green  dye  with  a  sharp  cut-off 
on  the  long-wave  side  combined  with  a  greenish  yellow 
dye  with  a  sharp  cut-off  on  the  short-wave  side  will 
yield  a  fairly  monochromatic  green  filter.  Some  of  the 
dyes  fluoresce  which  from  the  point  of  view  of  color  alone 
is  of  considerable  interest.  Fluorescein  and  uranine 
are  among  the  many  which  fluoresce  strikingly.  It  is 
interesting  to  study  these  by  projecting  a  spectrum  upon 
their  upper  liquid  surface  and  by  viewing  the  result 
both  from  above  and  from  the  side.  The  spectral  analyses 
of  potassium  bichromate  and  cobalt  chromate  are  in- 
cluded. 

Among  the  greens  in  Table  XXVII  are  a  number  of 
dichroics.  In  fact,  a  very  common  characteristic  of 
green  dyes  is  the  exhibition  of  dichromatism.  This  can 
readily  be  ascertained  by  noting  the  energy-spectrum 
or  spectral  transmission  characteristic  of  one  of  these 
dyes.  If  the  transmission-factor  for  red,  say  0.7^,  is 
in  any  one  case  greater  than  that  for  any  wave-length 
in  the  other  regions  of  the  spectrum  (in  the  green  for 
so-called  green  dyes)  the  solution  at  great  depths  or 
concentrations  will  appear  red  and  therefore  will  be 
dichroic.  Naphthol  green  is  an  excellent  yellowish  green 
dye.  Among  the  greens  presented,  malachite,  saurgriin, 
methylengriin,  and  neptune  green  exhibit  dichromatism. 

One  of  the  most  annoying  features  of  dyes  is  the 
extreme  rarity  of  pure  blue  dyes.  Nearly  all  blue  dyes, 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA          373 

Table  XXVIII,  transmit  the  extreme  red  rays  quite 
freely  and  the  scarcity  of  blue-green  dyes  which  are  not 
dichroic  makes  it  difficult  often  to  find  a  combination 
which  transmits  only  the  violet  rays.  In  extremely 
high  concentrations  or  great  depths  some  blue  dyes 
effectually  absorb  most  of  the  extreme  red  rays. 

In  Table  XXIX  are  presented  a  number  of  spectral 
analyses  grouped  under  the  common  name  of  purple 
for  the  purpose  of  classification.  An  interesting  case 
is  that  of  ethyl  violet  in  gelatine  both  wet  and  dry.  After 
the  dyed  gelatine,  which  was  flowed  on  clear  glass,  had 
set,  and  while  still  wet  the  spectral  analysis  was  made. 
The  sample  was  then  allowed  to  dry  and  another  spectral 
analysis  was  made.  On  plotting  these  data  a  decided 
difference  in  the  spectral  transmission  curves  is  seen 
as  indicated  by  the  numerical  data.  The  wet  specimen 
is  decidedly  more  reddish  than  when  dry  and  an  actual 
shift  in  the  absorption-band  takes  place  on  drying. 
Although  not  definitely  established  this  may  be  ex- 
plained as  due  to  a  difference  in  the  refractive-index 
of  the  solvent  in  the  two  cases.  The  data  are  corrected 
for  reflections  from  the  gelatine  and  glass  surfaces. 

In  Table  XXX  are  presented  spectral  analyses  of 
dyed  gelatine  filters  before  and  after  fading  by  exposure 
to  solar  radiation.  Such  data  are  of  special  interest  in 
many  cases  and  it  appears  of  interest  to  make  a  thorough 
study  of  the  fading  of  dyes  with  the  aid  of  spectral  analy- 
ses. Certainly  no  great  amount  of  information  is  avail- 
able regarding  the  relation  of  the  spectral  character 
of  radiation  to  the  spectral  deterioration  of  dyes  or  the 
relation  of  either  of  these  to  the  chemical  composition. 
Incidentally,  the  testing  of  dyes  under  illuminants 
containing  ultra-violet  rays  of  extremely  short  wave- 
lengths which  are  practically  absent  in  solar  radiation 
at  the  earth's  surface  or  in  artificial  illuminants  as  com- 


374 


COLOR  AND   ITS   APPLICATIONS 


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CERTAIN  PHYSICAL  ASPECTS   AND   DATA 


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COLOR    AND    ITS    APPLICATIONS 


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CERTAIN   PHYSICAL    ASPECTS    AND    DATA         377 

monly  encountered  is  open  to  criticism.  Spectral  analy- 
sis has  not  been  sufficiently  utilized  in  permanency  tests 
to  warrant  all  the  conclusions  which  have  been  drawn 
in  this  matter  although  some  excellent  work  has  been 
done.6  Mott  has  shown  that  the  results  with  the  (  snow- 
white  '  flame  arc  in  dye-fading  are  practically  the  same 
as  those  obtained  in  daylight.  He  states  that  the  white 
flame  arc  at  25  amperes  affords  light  at  a  distance  of  two 
feet  more  intense  than  summer  sunlight.  By  focussing 
the  image  of  a  quartz  mercury  arc  by  means  of  a  quartz 
lens,  an  intense  illumination  rich  in  ultra-violet  rays 
may  be  obtained.  The  large  incandescent  lamps  may 
also  be  used  with  success. 

95.  Applications  of  Spectral  Analyses  of  Dyes.  —  The 
uses  for  spectral  analyses  of  dyes  are  manifold,  as  in 
the  case  of  any  class  of  colored  media.  In  general,  they 
provide  a  physical  basis  for  systematic  color-mixture  be- 
sides providing  the  necessary  information  for  choosing 
dyes  for  many  purposes.  In  many  aspects  of  color- 
technology  only  the  integral  or  subjective  color  is  finally 
of  interest  but  the  author  cannot  refrain  from  empha- 
sizing that  even  in  such  cases  an  intimate  knowledge 
of  colored  media  and  their  mixture  cannot  be  attained 
without  spectral  analyses  and  that  the  combination  of 
dyes  becomes  systematic  with  such  data  available. 

With  spectrophotometric  apparatus  well  maintained, 
a  complete  spectral  analysis  can  be  made  in  about  an 
hour  although  there  is  much  room  for  improvement  in 
such  apparatus  which  will  result  in  the  saving  of  time. 
However,  this  is  not  a  serious  matter  because  for  a 
given  coloring  material  only  one  anlaysis  need  be  made, 
as  will  be  shown  later,  to  provide  information  for  all 
degrees  of  concentration  or  depth  of  solution.  The 
author  has  available  hundreds  of  spectral  analyses  which, 
after  once  obtained,  are  a  perpetual  source  of  information. 


378  COLOR  AND  ITS  APPLICATIONS 

96.  Laws  Pertaining  to  Colored  Solutions.  —  In  order 
to  simplify  the  study  of  coloring  media,  especially  dyes 
and  colored  glasses,  several  simplifications  have  been 
made.  These  are  based  on  theory  and  have  been  con- 
firmed by  experiment  on  a  few  typical  specimens.  In 
order  to  develop  this  procedure  it  is  necessary  to  revert 
to  some  of  the  established  laws.  Lambert  first  stated 
that  all  layers  of  equal  thickness  of  a  transparent  medium 
absorb  equal  fractions  of  the  radiant  energy  which  enters 
them.  This  is  true  for  homogeneous  or  monochromatic 
radiation,  but  cannot  be  applied  to  the  total  absorption 
of  radiant  energy  of  many  wave-lengths  or  of  extended 
spectral  character. 

It  follows  from  Lambert's  law  that  if  the  thickness 
of  the  absorbing  medium  increases  in  arithmetical 
progression  the  radiation  transmitted  should  decrease 
in  geometrical  progression. 

Let  J  be  the  intensity  of  radiation  of  a  given  wave- 
length entering  a  layer  dl,  then— 


On  integrating  this  we  obtain, 


where  J0  is  the  original  intensity,  J  the  intensity  after 
traversing  a  thickness  d,  and  k  is  a  constant  depending 
upon  the  substance  and  upon  the  wave-length  of  the 
radiant  energy.  Various  terms  have  been  applied  to 
this  factor  such  as  absorption-index.  In  logarithmic 
form  this  equation  is  expressed  as, 

Log  T  =  log  T\  =  -  k\d  log  e  =  -  e^d 

Jo 

where  7\  is  the  transmission-factor  for  energy  of  wave- 
length X,  and  the  subscripts,  X,  indicate  the  factors  which 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA          379 

vary  with  the  wave-length.  Beer  deduced  the  law  that 
the  absorption  is  the  same  function  of  the  concentration 
of  a  dispersing  absorbing  substance  as  of  the  thickness 
of  a  single  substance  which  may  be  expressed  thus : 

/  =  J0Axcd  or  Tx  =  Axcd  or  log  Tx  =  cd  log  Ax 

where  c  is  the  concentration,  A  is  the  transmission- 
coefficient  or  transmissivity  and  the  other  symbols  repre- 
sent the  same  factors  as  in  the  foregoing  equations. 
The  validity  of  Beer's  law  has  been  questioned  by  some 
and  it  appears  that  there  is  some  doubt  as  to  its  validity 
in  such  cases  as  colloidal  solutions.  This  law  appears 
to  hold  when  the  absorbing  power  of  a  molecule  is  un- 
influenced by  the  proximity  of  other  molecules.  Ob- 
viously, if  any  change  takes  place  in  the  condition  of  the 
dispersed  substance  on  altering  the  concentration  the 
law  will  not  hold.  Incidentally,  there  is  work  to  be  done 
on  the  validity  of  this  law  hi  the  cases  of  'colloidal' 
glasses.  Lambert's  law  appears  to  be  firmly  established. 

In  so  far  as  the  foregoing  laws  are  valid  (and  it  ap- 
pears that  this  is  true  for  all  practical  purposes  such 
as  described  in  this  chapter)  for  a  given  solution  long  T\ 
is  proportional  to  c?,  and  for  a  given  depth,  or  containing 
cell,  log  TX  is  proportional  to  c.  By  the  use  of  coordinate 
paper  having  a  logarithmic  scale  along  one  axis  and  a 
uniform  scale  along  the  other,  a  great  deal  of  interesting 
data  can  be  obtained  from  one  spectral  analysis. 

By  means  of  the  foregoing  mathematical  relations 
the  spectral  analyses  of  colored  solutions  (and  colored 
glasses)  of  any  thickness  and  concentration  can  be 
obtained  from  two  determinations  of  spectral  character 
which  may  be  reduced  to  a  single  determination.  Such 
a  method  has  been  found  exceedingly  practicable  in 
preliminary  reconnoitering  in  search  of  combinations 
of  dyes  for  filters,  in  the  development  of  colored  glasses, 


380  COLOR  AND  ITS  APPLICATIONS 

and  in  the  study  of  many  problems  arising  in  color- 
technology. 

Some  examples  will  suffice  to  illustrate  the  uses  of 
this  scheme  in  practice.  Assume  a  solution  of  methyl- 
engriin  of  either  known  or  unknown  concentration.  A 
cell  of  a  known  thickness  is  filled  with  the  solution 
and  a  spectral  analysis  is  made.  For  such  a  purpose  a 
fairly  low  concentration  or  small  depth  is  chosen  so  that 
radiations  of  all  wave-lengths  which  are  of  interest  are 
appreciably  transmitted.  On  logarithmic  paper  as  pre- 
viously described,  a  plot  is  made  as  shown  in  Fig.  139, 
the  transmission-factors  from  the  spectral  analysis  being 
plotted  on  the  logarithmic  scale  vertically  above  the 
arbitrarily  selected  point  on  the  abscissa  axis  in  this 
case  taken  as  unity.  The  abscissae  scale  may  represent 
either  concentration  or  depth  and  may  be  either  a  relative 
or  an  absolute  scale.  Straight  lines  are  drawn  through 
the  points  to  a  common  point  on  the  ordinate  axis 
representing  complete  transparency  or  unity  on  this 
logarithmic  scale.  This  is  the  common  point  if  correc- 
tions have  been  made  for  surface  reflections  in  the  cell 
or  from  the  glass  surfaces  in  the  case  of  a  colored  glass. 
If  these  corrections  have  not  been  made,  the  common 
point  usually  will  be  near  0.92  on  the  *  transmission  axis' 
if  two  surface  reflections  must  be  accounted  for.  Each 
straight  line  represents  the  relation  of  log  T\  and  depth 
or  concentration  for  a  certain  wave-length.  By  extend- 
ing these  lines  the  spectral  characteristic  of  any  depth 
or  concentration  may  be  read  from  the  corresponding 
vertical  line.  If  the  original  spectral  analysis  has  been 
made  with  care  such  a  simple  plot  yields  a  vast  amount 
of  data. 

97.  Dichromatism.  --Methylengriin  has  been 
chosen  in  Fig.  139  because  it  also  illustrates  the  inter- 
esting case  of  dichromatism  so  commonly  exhibited  by 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA 


381 


dyes.  It  is  seen  that  the  slope  of  the  line  for  0.72^  is 
less  than  any  of  the  others.  This  is  proof  that  the  dye 
is  a  dichroic.  Some  lines  are  very  steep  which  indicates 
a  large  value  of  the  extinction  coefficient  A  for  radiation 
of  these  wave-lengths.  From  the  plot  it  is  seen  that  this 
dye,  in  solutions  of  high  concentration  or  of  great  depth, 
will  not  be  green  but  will  be  red. 

Another  interesting  plot,  of  a  similar  nature  but  in- 


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Fig.  139.  —  Methylengriin  solution. 

cludhig  relative  luminosity-values  instead  of  transmis- 
sion-factors is  shown  in  Fig.  140  for  rosazeine.  The 
spectral  transmission-factors  for  the  spectral  analysis 
used  were  multiplied  by  the  visibility  of  radiation  in 
each  case  and  plotted  vertically  above  the  point  desig- 
nated by  unity  on  the  concentration  or  depth  scale. 
Instead  of  drawing  straight  lines  representing  various 
wave-lengths  to  a  common  point  on  the  ordinate  axis, 
each  line  is  drawn  to  a  point  of  this  axis  corresponding 
to  the  relative  visibility  of  radiation  of  the  particular 


382 


COLOR  AND  ITS  APPLICATIONS 


wave-length.  The  ordinate  axis  is  now  a  logarithmic 
scale  of  relative  luminosity.  By  extending  these  straight 
lines  a  graphical  picture  of  spectral  luminosity  of  the 
dye-solution  is  obtained  for  any  concentration  of  depth. 
It  is  seen  that  this  solution  in  great  depth  or  high 
concentration  becomes  deep  red  because  the  slopes 
of  the  lines  become  less  with  increasing  wave-length 


1.0 


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CONC£NTFf>tT/O/V    OR  &EPT/1 

Fig.  140.  —  Rosazeine. 

after  the  absorption-band  of  a  weak  solution  or  small 
depth  is  passed.  Incidentally,  it  will  be  noted  that  the 
slope  of  line  0.44/*  is  less  than  that  of  0.58^  which  shows 
that  in  low  concentrations  or  in  relatively  small  depths 
of  a  higher  concentration  the  solution  is  purple,  that  is, 
it  has  an  absorption-band  somewhere  between  0.44// 
and  0.60/z.  Only  a  few  wave-lengths  have  been  used 
for  the  sake  of  clearness. 

98.    Complete    Representation    of    the     Graphical 
Method.  —  In  reality  the  schemes  illustrated  in  Figs.  139 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA 


383 


and  140  are  only  completely  illustrated  by  means  of  a 
solid  of  which,  for  example,  Fig.  139  represents  a  pro- 
jection upon  the  face  of  the  solid  bounded  by  the  logarith- 
mic 'transmission'  scale  and  the  concentration  or  depth 
scale.  A  model  of  this  tri-dimensional  diagram  can  be 


Fig.  141.  —  Graphical  representation  of  laws  of  spectral  transmission. 

easily  made  and  should  be  instructive.  An  attempt  is 
made  in  Fig.  141  to  illustrate  the  relations  between 
transmission-factor,  wave-length  and  concentration  or 
thickness.  For  this  purpose  the  spectral  analysis  of  a 
thin  piece  of  gold  red  glass  was  chosen.  Many  of  the 
cross-section  lines  have  been  omitted  for  the  sake  of 
clearness.  The  scales  are  designated  and  the  thickness 


384  COLOR  AND   ITS  APPLICATIONS 

of  the  specimen  of  gold  ruby  glass  is  assumed  to  be 
2  units  on  the  relative  thickness  scale.  In  plane  2  repre- 
sented by  the  dash-dot  vertical  rectangle,  the  spectral 
transmission  is  shown  in  the  dash-dot  curve  a2b2c2. 
For  the  limiting  case  of  zero  thickness,  this  curve  be- 
comes a  straight  line,  T  =  1,  which  is  the  top  edge  of  the 
foremost  rectangle,  plane  0.  Several  points  of  the 
'master'  curve  in  plane  2,  were  taken  for  the  purpose 
of  illustrating  the  determination  of  the  spectral  char- 
acteristic of  the  glass  at  another  thickness.  In  this 
example,  thickness  5  units  is  taken  and  its  spectral  trans- 
mission is  shown  by  the  dotted  curve  hi  plane  5,  the 
farthest  vertical  rectangle.  This  curve  is  obtained  by 
drawing  straight  lines  in  the  'wave-length'  planes  from 
the  wave-lengths  on  the  upper  front  scale  through  the 
points  on  the  'master'  curve  in  plane  2  of  corresponding 
wave-lengths.  Thus  where  a  given  straight  line  inter- 
sects the  various  thickness  planes,  the  transmission- 
factors  for  that  wave-length  are  found.  For  example, 
&2  is  a  point  on  the  'master'  curve  in  plane  2  and  its 
value  as  read  from  the  transmission-scale  is  the  trans- 
mission-factor of  this  specimen  of  thickness,  2  units, 
for  radiation  of  wave-length  0.52^.  A  straight  line 
drawn  through  this  wave-length  on  T  =  1  and  through 
&2  (always  remaining  in  the  particular  wave-length 
plane)  when  prolonged  intersects  plane  5  at  bb  which 
is  the  transmission-factor  for  0.52/z  for  a  specimen  of 
the  same  glass  of  5  units  thickness.  Other  points,  ab, 
c5,  etc.,  are  found  in  the  same  manner. 

These  straight  lines  are  the  same  as  those  shown  in 
Fig.  148 ;  in  fact,  Fig.  148  would  be  seen  on  viewing  the 
solid,  Fig.  141,  from  the  righthand  side.  A  model  of 
this  solid  made  of  wires  and  painted  to  represent  the 
spectral  colors  should  be  instructive. 

In   Fig.    140   luminosity-values   were   treated   in   a 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA          385 

manner  similar  to  the  transmission-values  in  Fig.  139. 
These  also  can  be  completely  represented  by  a  solid 
in  a  manner  similar  to  that  shown  in  Fig.  141  excepting 
that  the  vertical  scale  must  represent  logarithms  of 
luminosity.  In  the  limiting  case  of  zero  thickness  the 
curve  will  not  be  the  upper  foremost  horizontal  line  but 
will  be  the  spectral  luminosity-curve  of  radiation  and 
will  lie  in  the  foremost  vertical  plane.  On  vie  whig  such 
a  solid  in  projection  from  the  proper  side,  Fig.  149  will 
be  seen  if  the  same  gold  ruby  specimen  be  taken  as  an 
example.  It  appears  unnecessary  to  illustrate  this 
possibility  since  the  general  procedure,  should  be  under- 
stood from  the  foregoing.  In  case  the  analysis  is  to  be 
made  for  a  particular  illuminant  the  limiting  curve  in  the 
foremost  vertical  plane  will  be  the  luminosity  curve  of 
the  illuminant. 

One  of  the  points  which  is  emphasized  in  dealing 
with  colored  media  in  the  foregoing  manner  is  that  the 
spectral  transmission-  and  reflection-factors  are  never 
zero  but  are  merely  relatively  low  for  some  wave-lengths 
as  compared  with  others.  This  is  often  forgotten  when 
spectral  analyses  are  made  with  instruments  because 
when  the  luminosity  falls  below  the  threshold  the  trans- 
mission-factor is  considered  to  be  zero;  however, 
the  threshold  depends  upon  the  intensity  of  illumination 
or  upon  the  brightness  of  the  light-source. 

99.  Spectral  Analyses  of  Glasses.  —  In  the  develop- 
ment of  colored  glasses  for  the  variety  of  practical  appli- 
cations, the  spectral  analyses  are  extremely  valuable 
and  often  essential.  By  means  of  such  data  these  color- 
ing elements  can  be  mixed  computationally  to  obtain 
the  desired  spectral  characteristic.  From  very  meagre 
data  on  the  chemical  composition  from  one  melt,  fairly 
definite  strides  toward  realization  may  be  made  in 
succeeding  melts.  Of  course,  there  are  chemical  con- 


386  COLOR  AND  ITS  APPLICATIONS 

siderations  which  sometimes  alter  the  predictions  based 
on  computation;  however,  such  a  procedure  forms  a 
most  definite  working  basis.  In  the  combination  of 
glasses  for  special  filters,  lighting  effects,  etc.,  the  com- 
putational method  often  saves  time  and  provides  definite 
data.  Sometimes  only  the  subjective  color  is  desired 
but  even  in  these  cases  spectral  analyses  of  elemental 
colorings  provide  the  basis  for  manipulating  the  avail- 
able verifiable  colored  media  in  a  manner  analogous 
to  the  combination  of  pigments. 

In  the  manufacture  of  colored  glass  there  is  a  limited 
number  of  coloring  materials  available  and  when  the 
glass  must  be  limited  to  one  general  composition,  such 
as  soda  lime,  for  example,  the  colors  which  are  possible 
of  attainment  are  further  limited.  However,  by  combin- 
ing various  coloring  materials,  the  variety  of  colored 
glasses  can  be  enormously  extended  to  meet  the  require- 
ments of  science  and  art. 

In  this  chapter  the  spectral  analyses  of  a  few  funda- 
mental colored  glasses  will  be  presented  and  also  the 
results  of  a  few  simple  combinations.  The  record  num- 
ber of  the  specimen  is  placed  before  the  symbol  of  the 
coloring  metal  such  as  37  Se.  If  different  relative  thick- 
nesses of  the  specimen  are  presented,  a  number  is  placed 
before  the  designation  proper  as  10(37  Se)  indicates 
10  units  of  thickness  (or  of  concentration);  CS  indicates 
lime  soda  glass;  PS,  lead  soda;  BS,  barium  soda;  P, 
lead,  etc. 

100.  Red.  —  Selenium,  copper,  and  gold  are  com- 
monly used  for  producing  red  glasses.  In  Fig.  142  are 
shown  the  spectral  analyses  of  a  number  of  selenium 
glasses.  It  is  seen  that  some  of  these  are  yellow  in 
appearance,  varying  from  this  to  a  deep  red.  The  com- 
position of  the  mix  is  sometimes  of  considerable  influence 
upon  the  final  color.  Specimen  14  Se  shown  in  relative 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA 


387 


thicknesses,  10,  20,  and  34  was  of  unknown  composition 
but  the  coloring  element  was  selenium.  This  is  a  re- 
markable specimen.  Cobalt  blue  glass  (Fig.  146)  trans- 
mits a  deep  red  band,  so  a  combination  of  dense  cobalt 
blue  and  selenium  glasses  isolates  a  deep  red  band  as 
seen  in  6  Co  +  14  Se,  Fig.  142. 

By  computations  similar  to  those  presented  hi  the 


.46        it8        .50         Jg        .34        J6         J8        .60         At        j64        46         Ad        JO 


Fig.  142.  — Red  Glasses. 

case  of  pigments  (substituting  transmission-factors,  7\, 
for  reflection-factors  R\)  the  efficiency  of  such  a  com- 
bination in  transmitting  only  a  deep  red  band  can  be 
compared  with  that  of  a  very  dense  selenium,  gold,  or 
copper  glass.  Unfortunately,  at  the  ends  of  the  visible 
spectrum  the  visibility  data  are  least  accurate;  however, 
such  relative  comparisons  by  computation  are  depend- 
able. Incidentally,  Hyde,  Cady,  and  Forsythe7  have 
determined  the  visibility  at  the  extreme  red  end  of  the 
visible  spectrum  with  great  care  and  Hartmann8  at 
the  blue  end. 


388 


COLOR  AND  ITS   APPLICATIONS 


In  Fig.  143  is  shown  the  spectral  characteristic  of  a 
copper  red  glass,  4  Cu,  and  in  Fig.  144,  the  spectral 
analyses  of  gold  glasses  are  presented.  Gold  produces 
a  beautiful  pink  in  low  concentrations  (or  hi  thin  layers) 
and  deep  red  in  the  higher  ones  (or  in  thicker  layers). 
The  absorption-band  is  seen  to  be  near  0.53/4  for  the  more 
transparent  glasses  and  it  is  interesting  to  note  glass 


F 


§?£*- 


r^3> 


Jt>4          M          M          .70       ,7Z 


Fig.  143.  —  Copper,  sulphur,  uranium,  and  chromium  glasses. 

35  Au,  a  lead  gold,  which  shows  a  shift  in  the  absorption- 
band  to  0.50/1.  This  glass  was  reheated  several  times 
in  bringing  out  the  color  which  is  decidedly  more  ruddy 
and  it  appears  that  there  is  a  different  state  of  division 
of  the  metallic  particles  perhaps  as  to  size.  As  the 
concentration  or  thickness  increases  (glass  5  Au,  which 
is  shown  for  three  thicknesses)  the  blue  band  gradually 
disappears;  however,  the  transmission  does  not  closely 
approach  monochromatism.  In  Fig.  144  are  also  shown 
the  results  of  combining  cobalt  and  gold  glasses  of  dif- 
ferent thicknesses  (or  concentrations),  with  the  resulting 
transmission  confined  to  the  deep  red  region. 


CERTAIN  PHYSICAL  ASPECTS  AND   DATA 


389 


101.  Yellow.  —  Carbon,  sulphur,  uranium,  and  silver 
are  among  those  elements  which,  when  introduced  into 
glass  under  proper  chemical  conditions,  produce  yellow 
glasses  of  varying  color.  No  single  element  isolates 
spectral  yellow.  In  Fig.  145  are  shown  the  spectral 
analyses  of  carbon  yellow  glasses,  15  C  and  44  C,  and  of 
combinations  of  carbon  yellow  and  light  cobalt  blue 


I 


11 


\ 


\ 


JO 


34         56        J9        .60        .6Z        jC>4        .66        AQ        JO 

tt&VC'  LCNGTH 


Fig.  144.  —  Gold  and  cobalt  glasses. 

glasses.  It  is  known  that  X-rays,  ultra-violet  and  visible 
rays  will  cause  some  clear  glasses  to  become  colored. 
In  Fig.  145  is  also  shown  the  spectral  characteristic  of  a 
glass  X,  which  though  originally  clear  was  colored  a 
muddy  yellow  throughout  the  mass  by  X-rays.  It  is 
interesting  to  observe  the  action  of  X-rays  in  discoloring 
glass,  for  it  is  easy  at  times  to  observe  the  progress  of 
the  coloring  through  the  thickness  of  the  glass.  Pat- 
terns can  be  made  by  this  process.  In  Fig.  143  are  shown 
the  spectral  characteristics  of  uranium  (11  U)  and  sul- 
phur (43  S  and  45  S)  glasses.  The  spectral  transmissions 


390 


COLOR  AND  ITS  APPLICATIONS 


of  several  uranium  samples  appear  to  be  kinky  in  the 
blue-green  region  although  the  exact  nature  of  the 
curves  are  not  established. 

102.  Green.  — Iron  imparts  a  green  color  to  glass 
varying  from  a  bluish  to  a  yellowish  green,  depending 
upon  the  ingredients  of  the  glass.  The  importance  of 
manganese  in  glass  is  as  a  decolorizing  agent,  its  color 


M         JO        JR 


Fig.  145.  —  Yellow  glasses  and  combinations  with  cobalt  glass. 

in  proper  concentrations  being  roughly  complementary 
to  that  of  iron  commonly  present  in  sand.  Chromium 
imparts  a  yellowish  green  color  to  glass  as  seen  in  glass 
53  Cr,  Fig.  143.  This  glass  has  a  maximum  transmission 
at  about  0.56/z  and  by  the  addition  of  copper  blue-green 
(glasses  2  Cu  and  8  Cu,  Fig.  143)  this  maximum  can 
be  shifted  toward  the  shorter  wave-lengths  depending 
upon  the  proportions  of  the  coloring  elements.  Glass 
21  Cu,  called  signal  blue-green,  is  evidently  a  copper 
glass.  Glass  36  Cr  is  a  dense  chromium  green.  In 
order  to  compare  the  actual  colors  under  a  given  il- 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA          391 

luminant  it  is  well  to  reduce  these  curves  to  luminosity 
values.  If  monochromatism  is  desired  it  is  often  ad- 
visable to  combine  two  glasses  which  transmit  a  narrow 
region  in  common. 

103.  Blue.  —  Cobalt  is  the  most  common  element 
used  to  impart  a  blue  co  or  to  glass.  Its  greatest  dis- 
advantage (although  sometimes  an  advantage)  is  its 


Fig.  146.  —  Cobalt  glasses. 

transmission  of  a  deep  red  band  as  shown  in  6  Co  and 
7  Co,  Fig.  146.  This  red  transmission  can  be  utilized  in 
isolating  the  deep  red  as  shown  by  combining  cobalt  and 
selenium  or  other  red  glasses,  for  example,  1  Co  +  14  Se. 
An  excellent  blue  glass  can  be  made  by  combining 
cobalt  with  copper  blue-green,  for  the  latter  effectively 
absorbs  the  deep  red.  The  spectral  characteristic  of 
such  a  combination  is  shown  in  9  Cu  +  6  Co,  Fig.  146. 
104.  Purple  —  Nickel  produces  a  purple  color  in 
glass  and  also  manganese  but  the  latter  is  not  an  efficient 
purple  because  its  absorption-band  is  not  sharp.  Its 


392 


COLOR  AND  ITS  APPLICATIONS 


chief  use  is  to  neutralize  the  green  tint  due  to  the  pres- 
ence of  iron  in  the  ingredients  of  glass  mixes.  The 
spectral  characteristic  of  a  glass  containing  iron  is  shown 
in  41  Fe,  Fig.  147.  It  is  seen  that  a  manganese  glass  of 
proper  density  is  approximately  complementary  in  color 
to  the  iron  glass.  Although  the  manganese  neutralizes 
the  iron  in  color,  the  transmission-factor  of  the  resultant 


ja      j6o 

MfrtT   LfHGTM 


JO      JZ 


Fig.  147.  —  Manganese  and  iron  glasses. 


glass  may  be  seriously  reduced.  Manganese,  though 
a  useful  element  in  glass  manufacture,  cannot  be  con- 
sidered important  as  a  coloring  element  from  the  view- 
point of  colored  glasses  in  general.  In  X,  Fig.  147,  is 
shown  the  spectral  characteristic  of  an  originally  clear 
glass  which  has  been  colored  a  deep  purplish  hue  by 
exposure  to  X-rays.  Undoubtedly  this  coloring  is  due 
to  an  effect  upon  the  manganese  present  in  the  clear 
glass.  This  effect  is  commonly  observed  in  lamp  globes 
and  window  glass  exposed  to  strong  sunlight.  In  the 
former  cases  it  is  a  very  serious  defect  of  glass  manu- 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA 


393 


facture  because  the  author  13  has  observed  such  globes 
whose  transmission  has  been  reduced  as  much  as  50  per 
cent,  after  long  exposure  to  intense  solar  radiation  or 
to  that  emitted  by  an  arc  lamp.  It  would  be  far  better 
in  such  cases  as  street-lighting  glassware  to  eliminate 
the  manganese  and  to  endure  the  unneutralized  greenish 
hue  of  the  iron  which  is  unavoidably  present. 


*3 

S-i 

$* 


\\ 


\\ 


\\ 


\ 


Fig.  148.  —  Gold  ruby  glass. 

105.  Use  of  Spectral  Analyses  of  Glasses.  —  The  ap- 
plications for  spectral  analyses  of  colored  glasses  have 
been  fairly  well  covered  in  the  discussions  of  pigments 
and  dyes,  for  the  same  general  procedures  can  be  applied 
to  colored  glasses.  The  concentration  is  not  so  definite 
as  in  the  case  of  dyes  because,  owing  to  the  high  temper- 
ature at  which  glass  melts  and  to  chemical  action,  the 
concentration  of  coloring  material  in  the  final  glass  can- 
not always  be  predicted  from  the  amount  of  coloring 
metal  added  to  the  mix.  Some  of  the  metals  such  as 
cobalt  and  copper,  under  standardized  conditions  of 


394 


COLOR  AND   ITS  APPLICATIONS 


melting,  appear  to  produce  concentrations  of  coloring 
material  proportional  to  the  amounts  of  the  oxides  added 
to  the  mix  but  in  some  cases  there  is  doubt  as  to  this 
proportionality.  There  is  need  for  systematic  study  in 


.VI 

.008 
J006 


.004 
.003 

.00* 


\ 


\ 


\ 


\ 


\\ 


\ 


\ 


\ 


\ 


Fig.  149.  —  Gold  ruby  glass. 

this  direction.  In  the  case  of  the  red  glasses,  for  example, 
gold  ruby,  which  in  ordinary  manufacture  assumes  its 
red  color  on  reheating,  the  manipulation  has  consider- 
able effect  upon  the  density  of  the  color.  After  a  colored 
glass  has  been  obtained  it  is  possible  to  procure  from  a 
single  spectral  analysis  the  integral  transmission-factor 
for  any  illuminant,  the  spectral  characteristics  of  other 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA 


395 


thicknesses,  and  those  of  combinations  of  these  thick- 
nesses with  other  colored  glasses  as  already  outlined. 

For  the  sake  of  further  exemplification,  in  Fig.  148 
are  shown  the  straight-line  relations  between  thickness 
and  transmission-factor  (for  entering  radiation)  for 
several  wave-lengths  for  various  thicknesses  of  a  gold 
ruby  glass.  The  relations  between  luminosity  and 


r 

I* 

.03 


O.S90 
0.&* 


CL65S 
Q666 


OF  GCJSS  /Af 

Fig.  150.  —  Test  of  '  straight-line '  law. 

thickness  for  this  glass  are  shown  in  Fig.  149  for  various 
wave-lengths.  Fig.  148  is  a  diagram  of  what  would  be 
seen  if  the  solid  represented  in  Fig.  141  were  viewed 
from  the  right-hand  side. 

At  this  point  it  is  of  interest  to  show  the  approximation 
of  experimental  results  to  the  relations  between  spectral 
character  and  thickness  as  predicted  by  theory.  This 
is  shown  in  Fig.  150  for  one  of  the  glasses  made  during 
the  development  of  an  '  artificial-daylight '  glass  some 
years  ago.  The  method  of  graphical  analysis  was  tested 
because  of  the  desire  for  a  simple  method  and  the  sped- 


396  COLOR  AND  ITS  APPLICATIONS 

men  was  ground  and  polished  in  five  thicknesses.  The 
circles  show  the  verification  of  the  theory.  In  this  case 
correction  had  not  been  made  for  surface  reflection 
so  the  straight  lines  must  be  drawn  to  a  point  near  0.92 
on  the  transmission-axis.  Incidentally,  it  is  of  interest 
to  note  that  previous  to  the  adoption  of  this  method, 
samples  of  melts  were  ground  hi  the  form  of  thin  wedges 
and  spectral  analyses  were  made  at  various  thicknesses. 
It  is  seen  that  the  graphical  method  enormously  reduces 
the  amount  of  work  in  order  to  obtain  the  data  neces- 
sary for  such  studies. 

In  developing  a  colored  glass  for  a  specific  purpose, 
various  factors  are  considered  such  as  the  illuminant 
to  be  used  and  the  result  to  be  obtained.  From  these 
an  ideal  spectral  transmission-curve  is  determined  and 
by  means  of  a  few  spectral  analyses  of  different  colored 
glasses,  bearing  in  mind  the  chemical  considerations 
if  a  mixture  is  finally  necessary,  various  combinations 
can  be  made  with  the  aid  of  the  graphical  method. 

Often  the  ultra-violet  and  infra-red  spectral  trans- 
missions are  of  interest  and  these  are  made  hi  the  man- 
ner already  described.  The  data  on  a  coloring  element 
is  not  considered  to  be  sufficiently  complete  for  record 
if  the  ultra-violet  transmission  is  not  studied  at  least 
qualitatively  and  in  some  cases  the  infra-red  transmis- 
sion is  investigated. 

106.  Influence  of  Temperature  on  Transmission  of 
Colored  Glasses.  —  Hyde,  Cady,  and  Forsythe 9  in  study- 
ing red  pyrometer-glasses,  noted  the  influence  of  temper- 
ature on  the  transmission  characteristic  of  a  red  glass 
and  investigated  this  influence  for  temperatures  from  20° 
to  80°  C.  The  transmission-factor  of  the  red  glass  was 
found  to  be  appreciably  less  for  various  wave-lengths 
at  the  higher  temperatures  than  at  the  lower  temper- 
atures. 


CERTAIN  PHYSICAL  ASPECTS  AND   DATA          397 

It  appeared  of  interest  to  ascertain  how  generally 
the  transmission-factors  of  colored  glasses  were  affected 
by  temperature.10  In  a  preliminary  study  it  did  not 
appear  worth  while  to  investigate  this  question  spectro- 
photometrically;  therefore,  only  the  transmission-factor 
for  total  visible  radiation  was  considered.  However, 
an  idea  of  the  change  in  spectral  transmission  is  gained 
through  the  change  in  color  of  the  specimen  as  its  temper- 
ature is  altered.  It  is  hoped  that  at  a  later  date  a  careful 
study  of  this  phenomenon  can  be  made  spectrophoto- 
metrically  and  in  parallel  with  chemical  investigations. 

In  order  to  eliminate  the  annoyance  of  large  color- 
differences  in  determining  the  transmission-factors  at 
different  temperatures,  a  given  specimen  was  cut  into 
two  pieces  and  one  was  kept  at  a  temperature  of  30°  C., 
while  the  temperature  of  the  other  was  altered  gradually 
from  this  temperature  to  350°  C.  The  transmission- 
factor  of  a  colored  glass,  of  course,  varies  with  the 
illuminant  so  that  such  a  value  is  indefinite  unless  the 
illuminant  is  specified.  In  this  account  it  appears  suffi- 
cient to  state  that  the  illuminants  used  were  gas-filled 
Mazda  lamps  operating  at  normal  voltage.  A  continuous 
check  on  the  constancy  of  the  light-sources  and  of  the 
transmission-factors  of  the  optical  paths  was  made 
possible  by  removing  the  two  colored  glasses  from  the 
optical  paths  momentarily  without  altering  the  temper- 
ature conditions.  The  relative  transmission-factors  of 
the  two  pieces  of  the  given  specimen  were  measured 
throughout  the  range  of  temperature  indicated  and  the 
results  for  ten  commercial  specimens  are  given  in  the 
diagram  and  in  Table  XXXI. 

No  color-difference  was  encountered  during  the  meas- 
urements except  that  due  to  a  change  in  the  spectral 
transmission  characteristic  of  the  heated  specimen. 
This  color-difference  became  very  marked  for  specimens 


398 


COLOR  AND  ITS  APPLICATIONS 


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CERTAIN  PHYSICAL  ASPECTS  AND  DATA          399 

5  and  10.  The  transmission-factor  of  the  hotter  piece 
is  given  in  terms  of  that  of  the  colder  piece  of  the  same 
specimen  at  the  various  temperatures  —  that  is,  the 
transmission-factors  as  given  are  relative  values  and  not 
absolute.  The  color  of  a  specimen  at  the  highest  temper- 
ature is  given  as  compared  with  that  of  a  piece  of  the 
same  specimen  at  30°  C.,  the  change  being  sufficient  to 
be  readily  described  in  terms  of  our  ordinary  indefinite 
terminology.  All  the  glasses  excepting  the  two  contain- 
ing cobalt  decreased  in  transmission-factor  as  the  tem- 
perature increased,  and  in  some  cases  this  decrease 
in  transmission-factor  was  very  large.  The  curves  ob- 
tained by  plotting  temperature  and  relative  transmission- 
factor  are,  in  general,  approximately  straight  lines  in- 
dicating that  throughout  this  range  of  temperature  the 
transmission-factor  changes  approximately  proportionally 
with  the  temperature  for  the  specimens  used.  Owing  to 
the  relatively  slight  change  in  hue  in  the  red  end  of  the 
spectrum,  the  red  glasses  1  and  9  did  not  change  ap- 
preciably in  color  when  heated,  notwithstanding  large 
decreases  in  their  transmission-factors. 

This  preliminary  study  indicates  an  interesting  field 
for  careful  research  which  might  throw  more  light  upon 
the  question,  'How  are  glasses  colored?'  It  will  be 
noted  that  the  highest  temperature  studied  is  below 
that  at  which  the  glass  becomes  self-luminous  or  plastic. 
It  will  be  of  interest  to  carry  this  investigation  close  to 
the  melting  point.  The  results  obtained  are  of  interest 
both  theoretically  and  practically. 

107.  Ultra-violet  Transmission.  —  Another  interesting 
fact  reported  by  the  author  n  is  the  increase  in  trans- 
mission of  clear  glass  for  certain  ultra-violet  rays  by 
the  addition  of  cobalt.  In  other  words,  the  range  of 
transmission  extended  further  in  the  ultra-violet  region 
in  the  case  of  the  cobalt  glass  than  in  the  case  of  the 


400  COLOR  AND  ITS  APPLICATIONS 

clear  glass,  the  slight  amount  of  cobalt  in  the  former 
being  the  only  difference  in  the  compositions  of  the  two 
glasses. 

Absolam  12  has  presented  the  data  in  Table  XXXII 
which  indicate  the  wave-lengths  where  complete  absorp- 
tion commences;  that  is,  in  each  case  the  wave-length 
indicating  the  longest  one  of  the  region  of  practically 
complete  absorption.  He  used  an  arc  between  copper 
poles  and  a  quartz  spectrograph. 
TABLE  xxxii 

Natural  blue  rock  salt Beyond  225/z/i 

Natural  rock  salt  colored  by  cathode  rays.    ......    .   ,  "      225 

Natural  rock  salt  colored  blue  by  cathode  rays .    .    .  '.    .    ..->  "      225 

Sylvite  white "      225 

Chile  saltpetre,  ordinary  white  variety .    .    .    .    ....;/.  351 

"        violet 325 

Fluorspar,  colored  deep  violet  by  cathode  rays Beyond  225 

Diamond,  yellow 320 

Diamond,  blue V  315 

Kunzite 305 

Garnet 402 

Zircon,  (hyacinth)   red-brown 262 

Zircon,  decolorized  by  heat  .    .    .,.    .    .    «    .    .  .... 244 

Zircon,  green •"•.••    •    • •  402 

Zircon,  yellow. 402 

Topaz,  pale  yellow.    .......:...... 262 

Topaz,  dark  yellow .    .    .    . '.    .    .    .    . 229 

Topaz,  pale  pink-brown 262 

Topaz,  blue 296 

Emerald 320 

Ruby 300 

Tourmaline,  green 351 

"          green-yellow 300 

"           pink 306 

Spinel,  blue .  402 

"     Purple 325 

"     Pink    .    .    . 300 

Kyanite,  blue 320 

Beryle,  blue 327 

Cordierite,   blue-purple 325 

Cairngorm       326 

Ordinary  clear  glass  is  practically  opaque  beyond 
300/z/z  although  clear  glasses  vary  considerably  in  trans- 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA          401 

parency  to  ultra-violet  depending  upon  the  content  of 
silica  and  other  ingredients.  In  general,  the  color  of  a 
substance  is  no  indication  of  its  transparency  to  invisible 
radiation. 

108.  Compounds  Sensitive  to  Temperature.  — Experi- 
ence with  the  effect  of  temperature  on  colored  glasses 
leads  to  the  belief  that  the  same  effect  would  be  found 
with  pigments  and  solutions.     In  fact,  some  of  these 
effects  have  been  noticed  and  it  would  be  of  interest 
to  investigate  this  point  systematically.     Certain  com- 
pounds change  color  with  change  in  temperature  and 
they  are  of  practical  as  well  as  of  scientific  interest. 

The  double  iodide  of  mercury  and  silver  is  normally 
a  light  yellow  but  its  color  changes  to  a  deep  orange  or 
red  at  about  50°  C.  Its  color  will  return  to  normal  on 
cooling  unless  it  has  been  overheated.  It  is  prepared 
from  separate  aqueous  solutions  of  silver  nitrate  and 
potassium  iodide.  The  latter  is  added  to  the  former 
until  the  original  precipitate  is  dissolved.  At  this  point 
a  strong  solution  of  mercuric  chloride  is  added  and  the 
precipitate  formed  is  the  bright  yellow  double  iodide  of 
mercury  and  silver.  This  is  filtered,  washed,  and  dried. 
It  may  be  used  as  a  paint  by  mixing  into  a  solution  of 
gum  arabic. 

The  double  iodide  of  mercury  and  copper  is  normally 
red  but  changes  to  black  at  about  85°  C.  returning  in 
color  to  red  as  it  cools.  This  is  prepared  in  a  manner 
similar  to  the  compound  in  the  preceding  paragraph 
excepting  that  copper  sulphate  is  substituted  for  silver 
nitrate. 

109.  Transmission    of    Water   and   Fog.  — The   se- 
lective scattering  and  consequent  selective  absorption 
of  the  atmosphere  is  well  known  and  is  illustrated  in 
Fig.  13.    The  fine  particles  of  dust  and  even  molecules 
of  gas  are  responsible  for  scattering  the  rays  of  shorter 


402  COLOR  AND  ITS  APPLICATIONS 

wave-length  more  than  those  of  longer  wave-length. 
For  this  reason  the  setting  sun  is  red ;  a  cloud  of  smoke 
is  blue,  and  the  shadow  of  a  puff  of  smoke  is  brownish. 
That  this  selectivity  is  dependent  upon  the  size  of  the 
particles  is  also  apparent.  For  example,  smoke  from 
the  tip  of  a  cigar  is  more  bluish  than  that  emanating 
from  the  moist  end.  In  the  latter  case  moisture  has 
condensed  around  the  carbon  nuclei  and  these  larger 
particles  do  not  scatter  light  so  selectively. 

It  is  also  known  that  water  appears  various  tints  of 
blue  and  blue-green  when  it  is  of  great  depth  and  purity. 
This  is  especially  noticeable  when  flying  over  bodies  of 
water  although  the  effect  of  the  color  of  the  bottom 
and  of  the  suspended  matter  washed  from  the  shore 
must  be  separated  from  that  of  the  water  alone.  Or- 
dinary observations  indicate  that  water  selectively 
transmits  rays  hi  the  green  region;  that  is,  rays  of 
wave-lengths  near  the  ends  of  the  visible  spectrum  are 
transmitted  less  freely  than  those  near  the  middle  or 
especially  in  the  green  region  —  between  0.5/z  and  0.6/z. 
This  seems  to  have  been  fairly  well  established  by 
experiment. 

Recently  Utterback  14  made  some  determinations  of 
the  passage  of  various  colored  lights,  obtained  by  means 
of  filters,  through  artificial  fogs  produced  by  expanding 
saturated  air.  His  results  indicate  that  his  fogs  were 
most  transparent  to  light  rays  of  wave-lengths  from 
0.53/z  to  0.59/-1.  The  transparency  decreased  rapidly 
toward  the  red  but  not  so  rapidly  toward  the  blue  end 
of  the  spectrum.  Abbott 15  obtained  similar  results 
for  water-vapor  when  there  were  dust  particles  hi  the 
air. 

Bertel  using  a  'submarine'  spectrograph  photo- 
graphed the  visible  spectrum  of  the  light  reaching  various 
depths.  His  results  show  the  visible  spectrum  to  be 


CERTAIN  PHYSICAL  ASPECTS   AND   DATA 


403 


rapidly  narrowed.  The  red  rays  being  totally  absorbed 
at  depths  of  5  to  10  meters;  the  orange  at  20  meters; 
and  the  yellow  at  100  meters.  In  the  other  end  of  the 
spectrum  little  selective  absorption  appears  to  take  place 
until  a  depth  of  30  meters  is  reached.  At  1700  meters 
no  light  has  been  detected  by  any  investigator.  The 
range  of  the  spectrograms  obtained  at  various  depths 
are  presented  in  Table  XXXIII. 

TABLE  XXXIII 


Depths  in  meters 

Range  of  wave-lengths 

2 

303/iM  to  TOOjuju 

10 

303     618 

20 

305     588 

30 

310     577 

40 

322     568 

50 

338     561 

60 

346     556 

70 

350     550 

80 

355     547 

90 

357     645 

100 

360     543 

200 

379     513 

300 

392     500 

400 

398     498 

Many  factors  can  influence  the  results  obtained  so 
that  there  is  bound  to  be  disagreement.  However, 
water  appears  to  have  a  definite  selective  transmission 
for  light  of  a  hue  in  the  neighborhood  of  green. 

110.  Color-temperature  of  Illuminants.  —  The  vari- 
ous colorimeters  and  the  spectrophotometer  have  been 
used  for  the  purpose  of  comparing  illuminants  and  of 
representing  their  spectral  characteristics  respectively. 
Another  method  is  to  compare  the  integral  color  of  an 
illuminant  (at  normal  operation  of  the  lamp)  with  the 
color  of  a  *  black-body'  radiator  and  rating  the  former 
in  terms  of  the  temperature  of  the  latter  when  a  color- 


404  COLOR  AND   ITS   APPLICATIONS 

match  (approximate  in  some  cases)  obtains.  In  Table 
XXXIV  the  results  obtained  by  Hyde  and  Forsythe  16 
are  presented  in  terms  of  absolute  temperatures  (Kel- 
vin scale).  These  values  are  termed  *  color  temper- 
atures.' From  these  data  and  those  on  the  black-body 
brightness-temperatures,  the  mean  brightness  of  each 
light-source  may  be  computed. 

TABLE  XXXIV 

Gas  flame,  fish-tail  (coal  and  water-gas) 1870  deg.  K. 

Hefner 1875 

Pentane,  10  c.  p.  standard 1914 

Candle,  paraffin 1920 

Candle,  sperm 1926 

Kerosene,  cylindrical  wick 1915 

Kerosene,  flat  wick .    .    .    ;   .    .    .    .  2045 

Acetylene,  as  a  whole 2368 

Acetylene,  one  spot 2448 

Nernst  glower,  2.3  w.  p.  h.  c 2388 

Carbon  filament,  4.0  w.  p.  h.  c 2070 

Treated  carbon  filament,  3.1  w.  p.  h.  c 2153 

Metallized  carbon  filament,  2.5  w.  p.  h.  c 2183 

Osmium  filament,  2.0  w.  p.  h.  c 2176 

Tantalum  filament,  2.0  w.  p.  h.  c 2249 

Tungsten  filament,  1.25  w.  p.  h.  c 2385 

Tungsten  filament,   0.9  w.  p.  h.  c 2543 

Tungsten  filament  (gas-filled),  0.5  w.  p.  h.  c. 2900   (approx.) 

Kingsbury17  using  some  of  the  foregoing  values  as 
reference  points  has  made  measurements  of  the  color- 
temperature  of  commercial  gas-burners  obtaining  values 
from  1940  to  2118  deg.  K,  As  is  to  be  expected,  the 
color-temperature  of  a  flame  is  within  certain  limits 
dependent  upon  its  shape,  size,  and  position  and  upon 
the  composition  of  the  gas.  This  method  of  rating 
illuminants  yields  valuable  results. 

REFERENCES 

1.  Phil.  Trans,  of  Roy.  Soc.,  A,  203,  p.  385. 

2.  Bull.  Bur.  Stds.,  9,  p.  283. 

3.  Proc.  Amer.  Soc.  Test.  Mat.,  1916,  17,  part  n. 


CERTAIN  PHYSICAL  ASPECTS  AND  DATA          405 

4.  Hyde,  Forsythe,  and  Cady,  Astrophys.  Jour.,  1918,  48,  p.  65. 

5.  Elec.  Wld.,  May  19,  1917. 

Jour.  Frank.  Inst.,  1918,  186,  p.  529. 
Jour.  Opt.  Soc.,  1919,  2-3,  p.  39. 

6.  Trans.  Amer.  Electrochem.  Soc.,  1915. 

7.  Astrophys.  Jour.,  1915,  42,  p.  285. 

8.  Astrophys.  Jour.,  1918,  47,  p.  83. 

9.  Astrophys.  Jour.,  1915,  42,  p.  302. 

10.  Jour.  Amer.  Ceramic  Soc.,  1919,  2,  p.  743. 

11.  Jour.  Frank.  Inst.,  1918,  186,  p.  111. 

12.  Phil.  Mag.,  1917,  33,  p.  452. 

13.  Gen,  Elec.  Rev.,  1917,  20,  p.  671. 

14.  Trans.  I.E.S.,  1919. 

15.  Annals  of  Astrophys.  Obs.,  3,  p.  214. 

16.  Jour.  Frank.  Inst.,  1917,  183,  p.  353. 

17.  Jour.  Frank.  Inst.,  1917,  183,  p.  781. 


INDEX  TO  AUTHORS 


Abney,  39,  68,  85,  93,  96, 104,  109, 190, 

297 

Aitken,  326 
Alcmaeon,  181 
Anaxagoras,  181 
Aristotle,  181 
Arons,  107 
Ashe,  132 
Aubert,  127,  143,  190 

Babbage,  226 

Baily,  223 

Baltzell,  318 

Becquerel,  214 

Bell,  130,  196 

Benham,  40 

Bloch,  106,  144 

Blondel  and  Rey,  144 

Boll,  187 

Bradford,  262,  320,  326 

Broca  and  Sulzer,  137, 144 

Brown,  223 

Briicke,  177 

Burch,  180 

Busstyn,  148 

Byk,  223 

Cady,  20 

Charpentier,  144 

Chevreul,  68,  79,  175,  307,  311 

Churchill,  147,  151 

Clutsam,  326 

Cobb,  123,  131 

Cohn,  262,  320,  326 

Crookes,  159 

Cros,  218 

Crova,  159,  197,  229 

Dember,  212 
Democritus,  181 
Diogenes,  181 
Donders,  186 


Dow,  132,  203 

Dufton  and  Gardner,  229,  241 

Ebbinghaus,  189 

Ebet,  326 

Edridge-Green,  124,  178,  188,  190 

Empedocles,  181 

Exner,  103 

Fabry,  107,  197 

Fechner,  39,  121 

Ferree,  208 

Ferry,  143 

Fery  and  Cheneveau,  197 

Fick,  143 

Fraunhofer,  18 

Garnett,  37 
Geissler,  127,  326 
Greenwood,  189 

Hagen,  85 

Hall,  343 

Harrison,  285 

Hauron,  du,  218,  223 

Haycraft,  146 

Helmholtz,  40,  116,  143,  171,  172,  177, 

180 

Hering,  172, 177,  178,  184 
Hertz,  6 
Hewitt,  44 
Holmgren,  151 
Houston,  199 
Hughes,  326 
Hussey,  229,  242 
Hyde,  E.  P.,  20,  90,  114,  143 
Hyde  and  Forsythe,  212 
Hyde,  F.  S.,  343 

Ives,  F.  E.,  102,  218,  221,  242 
Ives,  H.  E.,  20,  93,  103,  131,  146,  196, 
197,  204,  209,' 214,  217,  229,  238, 274, 
281,  285,  301,  305,  314,  323 


407 


40S 


INDEX   TO   AUTHORS 


Ives  and  Brady,  111,  245 
Ives  and  Kingsbury,  198,  212 
Ives  and  Luckiesh,  127,  202,  242 

Javel,  226 

Johnson,  223 

Joly,  60,  217,  218 

Jones,  B.,  258 

Jones,  L.  A.,  96,  98,  127,  237 

Jorgensen,  301 

Karrer,  199 

Kingsbury,  212 

Kirchhoff,  14 

Kirchhoff  and  Bunsen,  107 

Klein,  180 

Kleiner,  143 

Knoblauch,  45 

Koenig,  11, 101,  102, 181, 189, 199,  210 

Koenig  and  Brodhun,  120 

Koenig  and  Martens,  113 

Koenig,  £.,  223 

Koettgen,  229. 

Kries,  v.,  145,  183 

Kiihne,  187 

Ladd-Franklin,  186 

Lambert,  65 

Lea,  213 

Lehmann,  214 

Lippmann,  30,  214 

Loeser,  132 

Lucas,  197 

Luckiesh,  20,  53,  76,  88,  99,  130,  133, 
138,  143,  146,  152,  154,  158,  196, 
205,  207,  229,  239,  243,  245,  256,  260 

Luckiesh  and  Cady,  91,  229,  238 

Lumiere,  60,  219 

Lummer  and  Brodhun,  88,  108,  143 

MacDonald,  326 

MacDougal,  163 

Major,  326 

Martel,  301 

Maxwell,  6,  61,  73,  101,  124 

Mayer,  178 

Mees,  52,  229  • 

Merrill,  245 

Mie,  37 


Millar,  204 
Miller,  226 
Moore,  241,  258 
Morris-Airey,  207 
Mott,  343 
Munsell,  79 

Nagel,  180,  189 
Nernst,  197 
Neuhaus,  214 
Newton,  23 
Nicati,  190 
Nichols,  228 

Nichols  and  Franklin,  229 
Nichols  and  Merritt,  212 
Nicol,  32 

Nutting,  22,  88,  94,  95,  112,  121,  127, 
209 

Ostwald,  301 

Paget,  219 

Parry  and  Coste,  343 

Parsons,  159,  190 

Paterson,  307,  311 

Paterson  and  Dudding,  148 

Pfund,  212 

Pirani,  229 

Planck,  14 

Plateau,  143 

Plato,  181 

Porter,  145 

Prang,  82 

Preston,  22 

Priest,  212 

Purkinje,  11,  164,  191,  204 

Rasch,  197 

Rayleigh,  37,  124,  197 
Rice,  133 
Richtmeyer,  212 
Ridgeway,  85,  124 
Rimington,  312,  315,  322,  326 
Rood,  40,  68,  85 
Rowland,  29 
Ruchmich,  326 
Runge,  78 
Ruxton,  82 
Ryan,  257 


INDEX   TO    AUTHORS 


409 


Schanz  and  Stockhausen,  159 

Schwartzchild,  202 

Scriabine,  315 

Seebeck,  214 

Seig  and  Brown,  212 

Sharp  and  Millar,  243 

Shepherd,  221 

Simmance  and  Abady,  64 

Snellen,  131 

Stammer,  107 

Starcke,  223 

Stebbins,  212 

Steindler,  125 

Stefan-Boltzmann,  15 

Stevenson,  77 

Stuhr,  204 

Talbot,  143 

Thomson,  37 

Thorp,  217 

Titchener,  76,  262,  320,  326 

Toch,  343 

Torda,  212 

Townsend,  212 

Tschermak,  180 

Tyndall,  37 


Uhler  and  Wood,  49 
Uhthoff,  133 

Valenta,  214 
Vanderpoel,  301 
Vinci,  da,  177 
Voege,  159 
Vogel,  215,  229 

Weideman  and  Messerschmidt,  143 

Wien-Paschen,  14 

Wiener,  213,  214 

Whitman,  100 

Winch,  326 

Wollaston,  18 

Wood,  22,  47,  215,  291 

Wundt,  190 

Young,  26,  181 

Young-Helmholtz,  74,  101,  103,  172, 
181,  186 

Zenker,  214 
Zimmerman,  62 
Zindler,  85 


INDEX  OF   SUBJECTS 


Aberration,  chromatic,  118,  284 
Abney's  templates,  110 
Absorption,  35 

by  atmosphere,  147 

by  dust,  smoke,  304 

selective,  36 

spectra,  50,  51 
atlas  of,  52 

of  solid  dyes  and  refractive  index, 
309 

of  rhodamine  reflector,  45 
Acetic  acid,  334 
Acetone,  334 

Acetylene,  spectrum  of,  21 
Achromatic  lens,  119 
Acid  violet,  306 
Additive  disks,  papers  for  making,  63 

method  of  mixing  colors,  57 

primary  colors,  57 
Advertising,  colored  light  in,  278 

displays,  274 
Aesculine,  43,  202 

fluorescence  of,  43 

absorption  of  ultraviolet  by,  43 
Affective  value  of  colors,  262 
After-images,  170,  284 

colored,  171 

complementary,  170 

duration  of,  171 

effect  of,  170 

explanation  of,  172 

in  painting,  282 

negative,  170 

positive,  170 

production  of,  170 
Ah-  brush,  342 
Alcohol,  333 
Allegheny  County  Soldiers'  Memorial, 

257 
Amber,  336 

glass,  254 


Amyl  acetate,  334 

alcohol,  334 
Analysis  of  color,  86 
colored  media,  96 
color  of  illuminants 

by     photometer     and     color 

niters,  107 

by  colored  solutions,  107 
by     monochromatic      colori- 
meter, 97 
by   polarization    colorimeter, 

108 
by   trichromatic   colorimeter, 

105 
Aniline  dyes,  303, 328 

reflection  from  solid,  309 

powdered,  309 
Aniline  yellow,  57 
Anthracene,  43 

Appearance  of  colors  affected  by 
duration  of  stimulus,  163 
environment,  163,  307 
illuminant,  163,  285,  303,  Plate  IV 
intensity  of  illumination,  163 
size  and  position  of  retinal  image, 

163 

surface  character,  163 
retinal  adaptation,  163 
mercury  arc,  166 
Arc  spectrum,  17,  21 
Art  and  light,  285 
Art  galleries,  258,  294 

artificial  daylight  in,  244 
Artificial  daylight,  238 
and  the  colorist,  305 
production  of,  227,  230,  235 
uses  for,  234 
testing,  306 

versus  natural  light,  226,  227 
Artist,  aim  of,  282 
attitude  of,  282 


411 


412 


INDEX   TO    SUBJECTS 


Artist,  position  of,  282 

photography  and  the,  283 
terminology  used  by  the,  284 

Artists'  pigments,  328 

Art  of  mobile  color,  312 

Atmospheric  absorption,  147 

Auramine,  340 

Aurantia,  331 

Average  daylight,  228 

Balmain's  paint,  43 
Banana  oil,  334 
Benham  disk,  39 
Benzene,  334 
Benzine,  334 
Binocular  contrast,  177 
Black,  absolute,  72 
Black  body,  14 

radiation,  21 
Black,  bone,  333 

ivory,  333 

lamp,  333 

nigrosine,  333 
Black  paper,  72 

velvet,  72 
Blue,  cobalt,  298,  329 

Prussian,  329 

ultramarine,  298,  329 
Blue-green,  filter,  67      * 
Bone  black,  333 
Booth,  demonstration,  266 
Borax  bead,  309 
Brightness,  spectral  sensibility  and,  10 

contrast,  174,  Plate  m 

of  colors,  70 

effect  of  illuminant  on,  168 
Brightness  increment,  122 

scale,  81 

sensibility  of  retina,  122 
Brush,  air,  342 

Cadmium  yellow,  298,  331 

Calcium  fluoride,  41 

Canada  balsam,  336 

Carbon  dioxide  tube,  Moore,  241 

incandescent  lamp  spectrum,  21 
Carmine,  298,  332 
Cascade  method,  208 


Celluloid,  uses  of,  339 

coloring,  340 
Changeable  colors,  309 
Charts,  color,  82 
Chlorophyl,  43,  310 
Chrysoidine,  340 
Chromatic  aberration,  284 

of  eye,  118 
Chrome  yellow,  331 
Chromium  oxide,  298,  330 
Chromoscope,  218 
Clouds,     selective     transmission     of, 

38 
Cobalt  blue,  298,  329 

glass,  205 
Collodion,  336 
Colloidal  solutions,  37 
Color  analysis,  86 

of  illuminants,  97,  105,  107,  108 

of  media,  96 
Color  and  light,  23 

and  vision,  117 

blindness,  tests  for,  151 
Color  box,  Maxwell,  161 

chart,  Prang,  82 
Ruxton,  82 

codes,  317 

cylinder,  Chevreul,  79 

effects,    disappearing    and    chan- 
ging, 275,  278 
for  stage  and  displays,  272 
modern  tendencies  in,  276 
principle  of,  272 
spectacular,  257 
Color  harmony,  261 

in  decoration,  251,  257 

hi  glasses,  37 

in  interiors,  261  j 

in  lighting,  224 

in  north  rooms,  261 

in  south  rooms,  252 

matching,  302 
glasses,  308 
light,  309 
Color-mixing  apparatus,  60 

disk,  64 

Color  mixture,  54 
Color  music,  314 

suggested  hi  Nature,  319 


INDEX   TO    SUBJECTS 


413 


Color  names,  78 
notation,  77 
of  sun  altitude,  38 
phenomena  in  painting,  282 
Color  photography,  213 
Lippmann,  214 

Wood  diffraction  process,  215 
filter  processes  of,  218 
Joly,  218 
Paget,  219 
Lumiere,  219 
Shepherd,  221 
Ives,  221 
Kinemacolor,  222 
Kodachrome,  222 
Color  photometry,  191 
preference,  260,  320 
production  of,  23 
pyramid,  75 
sensation  curves,  104 
sensations,  growth  and  decay  of, 

137,  164 
produced  by  colorless  stimuli, 

39 

Color    sphere,    Runge,    78 
terminology,  69 
tree,  Munsell,  79 
triangle,  73 
vision  theories,  181 
wheel,  59 

Colored  fabrics,  appearance  of  wet,  310 
gelatine,  327 
glasses,  327 

analysis  of,  97 
for  eliminating  glare,  154 
for  protection  against  ultra- 
violet, 157 

for  use  with  field  glasses,  160 
for  varying  contrast,  160 
in  the  industries,  159 
tests  of,  157 
uses  for,  151 
Colored  headlights,  152 
lacquers,  327 
light  in  home,  252,  269 
lights  and  colored  objects,  273 
lights,  range  of,  148 
Colored  media,  analysis  of,  96 
papers,  328 


Colored  papers,  reflection  co-efficients 

of,  168 

under  colored  light,  273 
Zimmerman,  63 

Colored  patterns,  successive  contrast 
and,  173 

photographs,  projection  of,  218 

shadows,  269 

surroundings,  effect  of,  227,  245, 

250 
Colorimeter,  monochromatic,  93 

trichromatic,  101 

analysis  of  illuminants  by,  97, 105, 

107,  242 

Coloring  materials,  294,  327 
Colorless     stimuli,     color     sensations 

from,  39 
Colors,  283 

affective  value  of,  260,  320 

and  sounds,  312 

artistic,  262 

changeable,  309 

cool,  252 

emotive  value  of,  260,  320 

examination  of,  307 

Fechner,  39 

for  demonstration,  306 

in  Nature,  35,  54 

monochromatic,  35,  167 

of  feathers,  30 

of  fiery  opals,  28,  30 

of  insects,  30 

of  potassium  chlorate,  30 

pigment,  35 

produced  by  mixing  pigments,  56 

purity  of  spectral,  35 

two-component  mixtures  of,  99 

used  with  music,  317 

warm,  252 
Complementary  colors,  55 

filters,  57 

hues,  59 

spectral  hues,  59,  75 
Cones,  retinal,  120 
Congressional  library,  257 
Continuous  spectra,  16 
Contrast,  binocular,  177 

brightness,  174 

hue,  174 


414 


INDEX   TO    SUBJECTS 


Contrast,  in  Nature,  291 

in  pigments,  291 

in  paintings,  292 

simultaneous,  174,  285,  Plate  III 

successive,  173 

theories  of  simultaneous,  177 
Copal,  335 
Critical  frequency,  Porter's  law  of,  145 

wave  form  and,  146 
Crova's  method  of  photometry,  197 

solution,  197 
Crown  glass,  86 
Crystals,  30,  32 
Cyan  blue,  221 
Cyanine,  37,  303,  306 
Cylinder,  color,  79 

Dammar,  336 

Daylight,  artificial,  227,  230,  235,  305 

average,  228 

color  of,  38 

efficiency  of  illuminants,  231,  233 

testing  artificial,  306 

uses  for  artificial,  234 

variability  of,  228,  304 

versus  artificial  light,  225,  227 
Decoration,  color  in,  251,  257 
Defects  of  color  photography,  220 
Defining  power  of  eye,  283 
Demonstration  booth,  266 
Dichroic  dyes,  303,  306,  308 
Dichroism,  37 
Diffraction  26 

color  photography,  216 

grating,  26,  29 
copies  of,  29 
Rowland's,  29 
spectrum,  Plate  I 

Direct-comparison  photometry,  203 
Disk,  Benham,  39 

for  varying  brightness,  71 

for  varying  saturation,  71 

Maxwell,  61 

sectored,  90 

Whitman,  100 
Dispersion  of  glass,  25 

prismatic,  23 
Displays,  274 
Distribution  of  light  on  paintings,  291 


Durability  of  pigments,  342 
Dyes,  aniline,  328 
fluorescent,  310 

Ear,  analytic  ability  of,  313 

comparison  of  eye  and,  313 
Edridge-Green  theory,  187 
Effect,  Purkinje,  11 
Effects  of  radiant  energy,  7 
Efficiency,  daylight,  233 

lighting,  228 

radiant,  13 

Electromagnetic  theory  of  light,  6 
Electron,  6 

Emerald  green,  298,  330 
Environment,  and  colors,  303 
Emotive  value  of  colors,  320 
Eosine,  310 

pink,  303 

Equality-of-brightness  photometry,  203 
Erythrosine,  306 
Ether,  5,  334 
Ethyl  alcohol,  334 

violet,  37,  57,  63 
Extraordinary  ray,  33 
Eye,  116 

a  synthetic  instrument,  314 

chromatic  aberration  in,  118 

as  a  simple  lens,  283 

compared  with  ear,  314 

faults  of,  283 

not  analytic,  92,  313 

movements,  284 

optical  constants  of,  117 

Fabry's  solutions,  198 
Feathers,  color  of,  28,  30 
Fechner  coefficient,  121 

colors,  39 

law,  121 

Fibers,  transparency  of,  303 
Film,  celluloid,  339 

gelatine,  338 
Filters,  complementary,  57 

for  panchromatic  plate,  202 

for  ultraviolet  bands,  47,  51 

for  visible  rays,  47 

useful,  46 
Flashing  sign,  novel,  279 


INDEX   TO    SUBJECTS 


415 


Flicker  photometer,  Whitman-disk,  100 
Simmance-Abady,  64 

photometry,  203 
Flickering  lights,  139 
Flint  glass,  86 
Fluorescein,  43,  310 
Fluorescence,  41 

colors  and,  42 

effect  of  solvent  on,  45 

examination  of,  41 

excitation  of,  42 

in  color  matching,  308,  310 

tests  of,  310 
Fluorescent  dyes,  310 

reflector,  44,  153 

media,  43 
Fluorite  prism,  26 
Fluor  spar,  41 
Fovea  centralis,  184,  307 
Fraunhofer  lines,  18,  19,  Plate  I 
Frosting  solution,  337 

Gamboge,  298,  331 
Gelatine,  327,  334 

filters,  269,  337 
Glass,  color  of,  37 

crown,  86 

dispersion  of,  25 

flint,  86 

prism,  26 

transmission  of,  26 
Glasses,  colored,  154,  327 
Grain  alcohol,  334 
Grating,  diffraction,  26 

spectrum,  Plate  I 
Green  made  by  mixing  yellow  and  blue, 

299,  330 
Growth  of  color  sensations,  137,  142, 

164,  207 
Gum  kauri,  336 
Gum  water,  295 

Hauron  color  photography,  218 
Headlights,  green-yellow,  152 
Hefner  lamp,  spectrum  of,  21 
Hering  theory,  184 
Heterochromatic  photometry,  191 
Holmgren  test,  151 
Houston's  solutions,  199 


Hue,  70 

and  the  illuminant,  169,  286,  Plate 
IV 

contrast,  176,  Plate  HI 

difference,  minimum,  125 

sensibility,  124 
Huyghen's  principle,  5 

Iceland  spar,  32 

Illuminants,  brightnesses  of  colors  and, 
167 

misuse  of,  226 

simulating  old,  253 

spectra  of,  13 

temperature  and  color  of,  13 

values  and,  167 

Illusion  of  intense  illumination,  291 
Impressionism,  60 
Indian  red,  332 

yellow,  298,  331 
Indigo,  298,  330 
Induction,  175 
Infra-red,  opacity  of  water  to,  42 

•     photography,  47 
Insects,  color  of,  30 
Interference,  29 

constructive,  3 

destructive,  3 
Interiors,  color  in,  251 
Iridescent  crystals,  28,  29 
Irradiation,  179 
Isolating  spectral  lines,  47, 51 
Ives,  (F.  E.)  color  photography,  221 
Ivory  black,  333 

Joly  color  photography,  218 
Juxtapositional  method,  60 

Kerosene,  43 

Kinemacolor,  222 

Kodachrome  color  photography,  222 

Kries  (v.)  duplicity  theory,  183 

Lacquers,  336 

celluloid,  337 

colored,  327 

Ladd-Franklin  theory,  186 
Lakes,  332 


416 


INDEX   TO    SUBJECTS 


Lambert  color-mixer,  65 
Lamp  black,  333 
Laws  of  radiation,  14 
Law,  Bloch,  144 

Blondel  and  Rey,  144 

Porter,  145 

Talbot,  143 
Legibility  of  type,  137 
Lens,  achromatic,  119 

simple,  118 
Light  and  Art,  285 

color,  23 

Light  beam,  diagram  of,  31 
Light,  1 

definition  of,  1 

electromagnetic  theory  of,  6 

production,  12 

sensation,  7 

shade,  and  color,  282 

the  soul  of  art,  285 

velocity  of,  6 

waves,  4 

analogies  of,  3 

and  sound  waves,  313 

white,  9,  38 

Lights  of  short  duration,  143 
Lighting  artist,  285 

color  in,  224 

of  art  galleries,  258 

of  paintings,  286,  291 
Line  spectra,  16 
Linseed  oil,  334 

Lippmann  color  photography,  214 
Lumiere  color  photography,  219 
Luminosity  curve  of  eye,  208 

equation  for,  211 

Macula  lutea,  307 
Madder  pigments,  332 
Magenta,  221 
Malachite  green,  307,  331 
Martius  yellow,  331 
Mastic,  335 
Matching  of  colors,  302 

artificial  daylight  for,  305 
Maxwell  disks,  61 

color  triangle,  73 

color  box,  101 
Mercury  afrc,  spectrum  of,  17,  46,  50 


Mercury  arc,  visual  acuity  and,  131, 
136 

colors  under,  166 
Methods  of  color  photometry,  192,  208 

limitations  of,  193 

secondary,  196 
Methyl  alcohol,  333 

violet,  37,  303,  306 
Mica,  29 

Miscellaneous  notes,  341 
Mixture  of  colors,  54 

by  shadows,  66 

two-component,  99 
Mobile-color  art,  312 

development  of,  317 

future  of,  326 

instruments  for,  321 
Monochromatic  colors,  35, 167 

acuity  in,  135 
Moore  tube,  241 
Multiple  reflection,  36,  248,  308 
Music,  development  of,  312 

evolution  of,  318 
Musical  notation,  78 

Naphthol  green,  57,  63,  202 

yellow,  306,  331 
Naphthalin  red,  310 
Neodymium,  47 
Newton's  experiment,  23 

rings,  30 
Nicol  prism,  33 
Nigrosine,  307,  333 
Non-selective  brightness  control,  114 
Normal  spectrum,  26,  Plate  I 
Notation,  color,  77 
Novel  color  effects,  274 

Ochres,  332 

Old  illuminants,  simulating,  253 
Opal,  fiery,  28,  30 
solution,  337 
Oil  film,  29 
Ordinary  ray,  33 
Organic  dyes,  43 
Overhand  method,  310 

Painting,  after-images  hi,  173 
color  phenomena  in,  282 


INDEX  TO   SUBJECTS 


417 


Painting,  artificial  daylight  for,  286 

in  artificial  light,  287 
Paintings,  cleaning,  296 

hanging,  292 

lighting,  291,  294 
Paints,  294,  329 

phosphorescent,  341 
Panama-Pacific  Exposition,  267 
Papers,  colored,  328 

yellow  vs.  white,  226 
Paraifin  prism,  26 
Phloxine,  310 
Phosphorescence,  41,  340 
Photo-electric  cell,  196,  200 
Photography,  color,  231 

infra-red,  47 

the  artist  and,  283 

true  values  in,  201 

ultraviolet,  47 
Photometry,  color,  191,  207 

filters  for,  108 

primary  methods  of,  192 

secondary  methods  of,  196 
Pigments,  169,  294,  328 

characteristics  of,  298 

classes  of,  295 

contrast  by,  291 

durability  of,  297,  342 

limitations  of,  291 

mixing,  66,  297 

purity  of,  297,  299 

sources  of,  296 
Pitch  prism,  26 
Planck's  law,  14 
Plane  of  polarization,  31 

rotation  of,  34 
Plane-polarized  light,  31 
Polarization,  30,  31 

by  crystals,  32 

by  reflection,  31 
Polarized  light,  31 
Poppy  oil,  334 

Potassium  bichromate,  306,  331 
Preference,  color,  260,  320 
Primary  colors,  66,  67 
Primary  sensation  curves,  182 
Printing  inks,  328 
Prismatic  spectrum,  18,  Plate  I 
Prisms,  26 


Production  of  light,  12 
Prussian  blue,  265,  330,  340 
Purity  of  colors,  70 
Purkinje  effect,  11,  164, 191,  204 

reversed,  205 
Purple,  74, 167 

visual,  187 
Pyramid,  color,  75,  76 

Quartz,  dispersion  of,  25 
polarization  by,  32 
prism,  26 
transparency,  26 

Radiant  efficiency,  13 

energy,  7 
Radiation  and  light  sensation,  7 

and  temperature,  11 

from  a  solid,  8  • 

laws,  14 
Rainbow,  7,  24 
Range  of  colored  lights,  148 
Red,  332 

References,  22,  63,  68,  86,  114,  161, 
180, 189,  211, 223, 270,  281,  301,  311, 
326,  343 

Reflection,  selective,  36 
Reflectometer,  112 
Refraction,  23 
Refractive  index,  25 

absorption  of  dyes  and,  309 
Resins,  335 

solubility  of,  336 
Resorcin-blue,  310 
Retina,  brightness  sensibility  of,  122 

color  sensibility  of,  119,  307 
Retinal  rivalry,  177 
Rhodamine,  202,  303,  306,  310 

reflector,  44 
Rivalry,  retinal,  177 
Rock  salt  prism,  26 
Rods,  119 
Rose  bengal,  310 
Rotation  of  plane  of  polarization,  34 

Sandarac,  335 
Saturation  of  colors,  70 

sensibility,  127 
Scattered  light,  37 


418 


INDEX   TO    SUBJECTS 


Scattered  light,   colored  glasses  and, 

162 

Sectored  disk,  90,  114 
Seeing,  282 
Selective  absorption,  35,  38 

reflection,  28,  248 

scattering,  38 

transmission,  35,  38 
Sensation  curves,  primary,  182 
Sensibility,  brightness,  122 

hue,  119,  124 

retinal,  120 

saturation,  127 
Shades,  71 
Shadows,  colored,  66 

daylight,  304 

hi  painting,  291 
Shellac,  336 

Shepherd  color  photography,  221 
Shooting  glasses,  154 
Signaling,  146 

lights  for,  146,  152 
Silver  film,  48 

Simmance-Abady  photometer,  64 
Simultaneous    Contrast,     174,     Plate 

m 

instantaneity  of,  178 

in  color  matching,  307 

hi  painting,  285     . 
Skylight,  color  of,  38 

origin  of,  38 

spectrum  of,  21 

natural,  304 

artificial,  305 
Slit  of  spectroscope,  24 
Smoke,  absorption  by,  38 
Soap  bubbles,  30 
Solar  spectrum,  17,  18 
Solutions,  Crova,  197 

Fabry,  198 

Houston,  199 

Ives  and  Kingsbury,  198 

Karrer,  199 
Solvents,  333 
Sounds  and  colors,  312 
Spectra,  arc,  17 

of  gases,  16 

of  illuminants,  13,  20,  21 

representative,  17 


Spectra,  of  solids,  16 

ultraviolet,  50,  51 

Spectral  character,  influence  of,  167, 
286 

colors,  35 

complementaries,  75 

distribution  of  energy,  20,  21 

lines,  19 

sensibility  of  eye,  10 

transmission  of  media,  91 
Spectrophotometer,  69,  88 

simple,  92 

portable,  89 
Spectroscope,  86 

direct  vision,  86 

accessories  for,  87 

comparison,  88 
Spectrum  analysis,  15 
Spectrum  of  daylight,  17 

helium,  17 

mercury,  17,  45,  50 

of  sodium,  48 

of  tungsten,  17 
Spectrum,  energy,  8 

grating,  26,  Plate  I 

normal,  26,  Plate  I 

production  of,  24 

rotating  colored  disk,  68 

visible,  8 

total,  8 

Specular  reflection,  309 
Sphere,  color,  78 
Spherical  light  waves,  5,  26 
Stage,  color  effects  for,  272 
Standardization  of  colors,  84 
Standing  wave,  3 
Stefan-Boltzmann  law,  15 
Subtractive  disks,  63 

color-mixing,  54,  298 

primary  colors,  55 
Subjective  yellow,  48 
Successive  contrast,  173 
Sunlight,  38,  304 

artificial,  239,  305 
Surface   character,   influence   of,   36, 

169,  302 

Surface  color,  309 

Surroundings,  influence  of,  245,  304, 
Plate  IH 


INDEX   TO    SUBJECTS 


419 


Talbot's  law,  143 

Tartrazine,  202,  331 

Temperature,  color  of  light  and,  9,  13 

radiation  and,  11 

spectrum  and,  9 
Templates,  109 
Terminology,  69 
Terra  verte,  298,  330 
Theory  of  color  vision,  181 

Edridge-Green,  187 

Hering,  184 

v.  Kries,  183 

Ladd-Franklin,  186 

Young-Helmholtz,  181 
Thinner,  295 
Tints,  71 
Tourmaline,  32 
Transmission,  35 

glass,  26 

selective,  36 

quartz,  26 
Tree,  color,  79 
Triangle,  color,  73,  76 
Tri-color  method,  73 
Tungsten  lamps,  spectrum  of,  21 
Turpentine,  335 

Venice,  335 

Ultramarine  blue,  265,  298,  329 
Ultraviolet  transmission  of  media,  50 

spectra,  50 
Uranin,  43,  57,  310 
Uranium  glass,  42 
Uviol  blue  glass,  42 

Value  scale,  81 
Values,  70,  283 

illuminants  and,  167,  286 

lighting  and,  286 
Varnish,  295,  335 
Vehicles,  295 
Velocity  of  light,  6 
Venetian  red,  332 


Venice  turpentine,  335 
Vermilion,  298,  332 
Visibility  of  radiation,  209 

of  point  sources,  149 
Vision,  278 

color  and,  116 
Visual  acuity  in  colored  light,  129,  135 

field,  120 

luminosity  filter,  199 

phenomena  in  painting,  282,  284 

in  color  matching,  302 
Visual  purple,  187 

bleaching,  188 

extracting,  177 
Visual  yellow,  188 

Wall  covering  for  paintings,  294 
Wave  motion,  2 

analogies  of,  3,  5 
Wave  theory,  1 

Welsbach  mantle,  spectrum  of,  21 
Wheel,  color,  59 
White  lead,  333 
White  light,  9,  38 

aesthetic,  265 

artificial,  304 

standard,  303 

subjective,  55,  235 
Wien-Paschen  law,  15 
Wood  alcohol,  333 
Wood  color  photography,  215 
Wundt  colored  papers,  63 

Yellow  pigments,  331 

solutions,  48 

spot,  307 

visual,  188 

versus  white  paper,  226 
Young-Helmholtz  theory,  101,  181 
Young's  double  slit  expt.,  26 

Zinc  chromate,  331 
white,  333 


-LIBRARY 


YC  '40017 


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wHICH  BORHOWED 

LOAN  DEPT. 


62A-50m-2,'64 
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.  Genera]  Library 

University  of  California 

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